Add incremental RST-to-MyST swap mechanism (circinus) (#1867)

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

Backport of the swap mechanism from feat/incremental-myst-swap onto
the circinus release branch. Built directly on top of origin/circinus,
so the underlying RST tree is circinus'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 (all from origin/myst/circinus):
- 253 md-prefixed pages alongside each {name}.rst counterpart
- 1 plain MyST-only page kept at canonical name (docs/copyright.md,
  no .rst counterpart)
- 182 .webp images added (circinus release previously had only
  PNG/JPG; this PR brings webp into circinus alongside the originals)
- docs/_swap.txt populated with all 253 stems → MyST is served by
  default; revert a page by removing its stem from _swap.txt

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

* fix(ext): handle RST fallback in CmdInclude when _renderer absent

`cmdincludemd` is in `myst_fence_as_directive`, so MyST routes
fence blocks through `render_fence → render_restructuredtext →
MockRSTParser`. In that path `self.state` is a plain docutils Body
with no `_renderer`, crashing the RTD build.

Fall back to `nested_parse` when `_renderer` is unavailable so the
directive works in both MyST and RST/MockRSTParser contexts.

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

* 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)

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

* docs: port .readthedocs.yml jobs and swap-script tests from PR #1857

Parity backport from PR #1857 (current) — was missing on circinus.

- .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.
- tests/test_import_myst.py, tests/test_swap_sources.py: tests for the
  swap scripts. The scripts are 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(review): add conftest, filter md- from copy, fix _prefer_webp builder list, trailing newline

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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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