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

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