Add incremental RST-to-MyST swap mechanism (sagitta) (#1868)

* feat(swap-sagitta): add incremental RST-to-MyST swap mechanism

Backport of the swap mechanism from feat/incremental-myst-swap onto
the sagitta release branch. Built directly on top of origin/sagitta,
so the underlying RST tree is sagitta's (not current's).

Mechanism:
- scripts/import_myst.py — import md from myst/* with md- prefix
- scripts/swap_sources.py — rename md-{name}.md → {name}.md before
  Sphinx builds, restore after; writes _build/_swap_state.json and
  _build/_swap_exclude.txt
- docs/Makefile — html/dirhtml/pdf/livehtml all run swap → build →
  trap restore; explicit `swap` and `restore` targets too
- docs/conf.py — MyST extensions enabled; swap exclude_patterns
  loader; _prefer_webp builder hook so html prefers webp over png

Content:
- 202 md-prefixed pages from origin/myst/sagitta (md-{name}.md
  alongside each {name}.rst counterpart)
- 1 plain MyST-only page from myst/sagitta where no .rst exists
  (already at canonical name on sagitta: docs/copyright.md)
- 240 .webp images from myst/sagitta (added alongside the existing
  PNG/JPG so RST builds keep their assets)
- docs/_swap.txt populated with all 202 stems → MyST is served by
  default, revert a page by removing its stem from _swap.txt

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* feat(conf): copy .md sources into HTML output for plain-text serving

Adds a build-finished hook that mirrors every .md file from the Sphinx
source tree into the HTML output directory verbatim, making unrendered
MyST sources accessible alongside HTML renders at the same URL path.

🤖 Generated by [robots](https://vyos.io)

* docs: address review feedback (backport from PR #1857)

Fix conversion artifacts, typos, and technical inaccuracies applicable
to the sagitta branch: curly quotes, typos (deamonless, cammans,
amdifferent, trough), incorrect firewall command paths, missing closing
brace in zone-policy, peer name inconsistencies, hardcoded passwords
replaced with vault references, and md-*.md exclusion in conf.py.

🤖 Generated by [robots](https://vyos.io)

* docs: port .readthedocs.yml jobs, _ext/vyos.py fallback and swap-script tests from PR #1857

Parity backport from PR #1857 (current) — three pieces were missing on
sagitta.

- .readthedocs.yml: add build.jobs.pre_build / post_build hooks that run
  scripts/swap_sources.py --swap before the Sphinx build and --restore
  after. Without this, the swap mechanism ships but never runs on RTD
  builds for this branch — the swap is a silent no-op.
- docs/_ext/vyos.py: CmdInclude.run() now falls back to nested_parse()
  when self.state._renderer is not present. Required for cfgcmd /
  opcmd / cmdincludemd directives to render correctly when included
  from MyST pages (the swap mechanism's whole point). Sagitta-only
  delta on _ext/vyos.py (the path = str(path) line on 224) is
  intentionally untouched.
- tests/test_import_myst.py, tests/test_swap_sources.py: tests for the
  swap scripts. The scripts on this branch are byte-identical to
  current's, so the same tests apply. Travels with the branch so CI
  catches per-branch regressions if the scripts ever drift.

🤖 Generated by [robots](https://vyos.io)

* fix(conf): skip md-*.md staging files in _copy_md_sources

Agent-Logs-Url: https://github.com/vyos/vyos-documentation/sessions/919695a7-688d-41b9-89f0-540684625dbc

Co-authored-by: andamasov <12631358+andamasov@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: andamasov <12631358+andamasov@users.noreply.github.com>

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