feat: import MyST swap mechanism + content for sagitta (replaces #1886)

Replaces the broken #1886 with a fresh, properly-converted MyST set for
the sagitta (1.4.x) docs, mirroring what landed for circinus via #1897.

This PR:

- Re-imports 210 md-*.md files for sagitta. Source: ran the pipelines
  rst-to-myst converter (chrisjsewell/rst-to-myst v0.4.0, with pandoc
  fallback) on sagittas RST. Post-processed via the pipelines
  postprocess stage (10 ordered fixes for blanks, admonitions, label
  hyphens, pandoc artifacts, structural blanks, linter markers).
  Compared to the broken #1886 content (which was left over from an
  earlier stage-1-only run): zero raw `<div class=>` remnants.

- For 23 stems where sagittas RST is byte-identical with currents RST
  (mostly stable policy/protocol pages and the 404 page), reuses currents
  already-validated md-*.md content rather than re-converting.

- Drops cli and installation/cloud/aws from sagittas swap set: their
  RST has SEVERE/4 "Title level inconsistent" errors that crash
  rst-to-myst; they need an independent RST-source fix and are kept as
  RST-only for now.

- Adds the per-page swap mechanism: scripts/swap_sources.py,
  scripts/import_myst.py, the matching tests under tests/, _swap.txt
  with 210 stems, _ext/vyos.py MyST renderer fallback, Makefile
  swap-wrapped targets, .readthedocs.yml swap pre/post hooks.

- Adds 187 .webp images and removes 235 superseded .jpg/.png/.jpeg
  static assets; flips html_logo to vyos-logo.webp.

- Adds the MyST swap-related blocks to docs/conf.py only:
  myst_enable_extensions, myst_fence_as_directive, md-*.md exclude
  patterns, _swap_exclude.txt reader, _prefer_webp and _copy_md_sources
  setup hooks. github_version fallback set to 'sagitta' to match the
  branch (parallel to currents 'current' and circinuss 'circinus').

Deliberately excluded (per user direction):

- llms.txt and sphinx-llms-txt / sphinx-sitemap config: these will
  land separately for sagitta via #1870 plus a new sagitta-specific
  llms.txt template PR. The conf.py here does not pull those extensions
  in, so the build does not depend on the new pip packages.

Verification before pushing:

- 210 md-*.md = 210 _swap.txt stems = 210 RST siblings on sagitta (1:1:1).
- 0 files contain raw `<div class=` (the breakage that took down /en/1.5/).
- conf.py copyright/version/release preserve sagittas values
  (2024 / 1.4 / "1.4.x (sagitta)") - not currents.
- html_title from currents conf.py removed - PR #1880 is the right place
  for sagittas branch-localized title.

Supersedes / closes on merge:

- #1886 (broken converter output, would break /en/1.4/ if merged).

Generated by robots https://vyos.io

2 files changed, 149 insertions, 0 deletions

diff --git a/docs/introducing/md-about.md b/docs/introducing/md-about.md
new file mode 100644
index 00000000..16eac6e5
--- /dev/null
+++ b/docs/introducing/md-about.md
@@ -0,0 +1,25 @@
+(about)=
+
+# About
+
+VyOS is an open source network operating system based on Debian GNU/Linux.
+
+VyOS provides a free routing platform that competes directly with other
+commercially available solutions from well known network providers. Because
+VyOS runs on standard amd64, i586 and ARM systems, it is able to be used
+as a router and firewall platform for cloud deployments.
+
+We use multiple live versions of our manual, hosted thankfully by
+<https://readthedocs.org>. We will provide one version of the manual for every
+VyOS major version starting with VyOS 1.2 which will receive Long-term support
+(LTS).
+
+The manual version is selected/specified by it's Git branch name. You can
+switch between versions of the documentation by selecting the appropriate
+branch on the bottom left corner.
+
+VyOS CLI syntax may change between major (and sometimes minor) versions. Please
+always refer to the documentation matching your current, running installation.
+If a change in the CLI is required, VyOS will ship a so called migration script
+which will take care of adjusting the syntax. No action needs to be taken by
+you.
diff --git a/docs/introducing/md-history.md b/docs/introducing/md-history.md
new file mode 100644
index 00000000..b400455f
--- /dev/null
+++ b/docs/introducing/md-history.md
@@ -0,0 +1,124 @@
+(history)=
+
+# History
+
+## In the beginning...
+
+There once was a network operating system based on Debian GNU/Linux,
+called Vyatta. [^footnote-1] 2006 onwards, it was a great free software
+alternative to Cisco IOS and Jupiter JUNOS. It came in two editions:
+Vyatta Core (previously Vyatta Community Edition) that was completely
+free software, and Vyatta Subscription Edition that had proprietary
+features and was only available to paying customers. [^footnote-2]
+
+Vyatta was acquired by Brocade Communication Systems in 2012. Shortly
+after, Brocade renamed Vyatta Subscription Edition to Brocade vRouter,
+discontinued Vyatta Core and shut down the community forum without a
+notice. The bug tracker and Git repositories followed next year.
+
+It's worth noting that by the time Brocade acquired Vyatta,
+development of Vyatta Core was already stagnated. Vyatta Subscription
+Edition (and thus, Vyatta development as a whole) had been replacing
+core components with proprietary software, meaning few features made
+it to Vyatta Core, and those that did were bug-ridden and hamstrung.
+
+In 2013, soon after Vyatta Core was abandoned, the community forked
+the last Vyatta Core version (6.6R1) and VyOS Project came into being.
+[Sentrium SL](https://blog.vyos.io/sentrium-what-sentrium) was
+established by VyOS maintainers in 2014 to fund VyOS development by
+selling support, consulting services and prebuilt long-term support
+images.
+
+Brocade was acquired by Broadcom in 2016 and sold what remains of
+erstwhile Vyatta to AT&T in 2017, who in turn sold it to Ciena in 2021.
+
+## Major releases
+
+VyOS major versions used to be named after elements in order of atomic
+numbers. With 1.2, this naming scheme was replaced with the much
+cooler scheme of Latin names of [IAU](https://en.wikipedia.org/wiki/IAU_designated_constellations_by_area)
+designated constellations by solid angle area, starting from the smallest.
+
+### Hydrogen (1.0)
+
+Released just in time for holidays on 22 December 2013, Hydrogen was
+the first major VyOS release. It fixed features that were broken in
+Vyatta Core 6.6 (such as IPv4 BGP peer groups and DHCPv6 relay) and
+introduced command scripting, a task scheduler and web proxy LDAP
+authentication.
+
+### Helium (1.1)
+
+Helium was released on 9 October 2014, exactly on the day VyOS Project
+first came into being in the previous year. Helium came with a lot of
+new features, including an event handler and support for L2TPv3,
+802.1ad QinQ and IGMP proxy, as well as experimental support for VXLAN
+and DMVPN (the latter of which was also broken in Vyatta Core due to
+its reliance on a proprietary NHRP implementation).
+
+### Crux (1.2)
+
+Crux (the Southern Cross) came out on 28 January 2019 and was the
+first major release of VyOS as we know it today. The underlying
+Debian base was upgraded from Squeeze (6) to Jessie (8).
+
+Although Crux came with too many new features to mention here, some
+noteworthy ones are: an mDNS repeater, a broadcast relay,
+a high-performance PPPoE server, an HFSC scheduler, as well as support
+for Wireguard, unicast VRRP, RPKI for BGP and fully 802.1ad-compliant
+QinQ ethertype. The telnet server and support for P2P filtering were
+removed.
+
+Crux is the first version to feature the modular image build system.
+CLI definitions began to be written in the modern, verifiable XML
+templates. Python APIs were introduced for command scripting and
+configuration migration. Introduction of new Perl and shell code was
+proscribed and the rewriting of legacy Perl code in pure Python began
+with Crux.
+
+As of 2022, Crux is still supported and maintained.
+
+### Equuleus (1.3)
+
+The current long-term support version of VyOS, Equuleus (the Pony)
+came out on 21 December 2021, once again in time for the winter
+holidays.
+
+Equuleus brought many long-desired features with it, most notably
+an SSTP VPN server, an IPoE server, an OpenConnect VPN server and
+a serial console server, in addition to reworked support for WWAN
+interfaces, support for GENEVE and MACSec interfaces, VRF, IS-IS
+routing, preliminary support for MPLS and LDP, and many other
+initialisms.
+
+As of 2022, Equuleus is in the stable.
+
+### Sagitta (1.4)
+
+Sagitta (the Arrow) is the codename of the current development
+branch, so there's no VyOS 1.4 yet.
+
+### Circinus (1.5)
+
+Circinus (the Compass) is the codename of the upcoming development
+branch, so there's no VyOS 1.5 yet.
+
+## A note on copyright
+
+Unlike Vyatta, VyOS never had (nor will ever have) proprietary code.
+The only proprietary material in VyOS is non-code assets, such as
+graphics and the trademark "VyOS". [^footnote-3] This means you can build your
+own long-term support images (as the entire toolchain we use is free
+software) and even distribute them, given you rename it and remove
+such assets before building. Although note that we do not provide
+support for images distributed by a third-party. See the
+[artwork license](https://github.com/vyos/vyos-build/blob/current/LICENSE.artwork)
+and the end-user license agreement at `/usr/share/vyos/EULA` in
+any pre-built image for more precise information.
+
+[^footnote-1]: From the Sanskrit adjective "Vyātta" (व्यात्त), meaning opened.
+
+[^footnote-2]: A business model comparable to that of Redis, rather than that
+    of VyOS today.
+
+[^footnote-3]: This is not unlike how Linus Torvalds owns the trademark "Linux".