feat: import MyST swap mechanism, llms.txt feature, and content from current

Replaces the broken #1885/#1869/#1875 trio with a single PR for circinus.

This PR:
- Imports 254 md-*.md files from currents working MyST conversion (the
  broken converter that produced #1885s md-*.md files is not used here;
  currents files are correct as verified on /en/rolling/).
- Adds the per-page RST-to-MyST swap mechanism: scripts/swap_sources.py,
  scripts/import_myst.py, tests, _swap.txt, _ext/vyos.py and Makefile
  swap-wrapped targets, .readthedocs.yml pre/post hooks.
- Replaces 175 .jpg/.png with 187 .webp images for swapped pages.
- Adds the llms.txt + sitemap feature (sphinx_llms_txt, sphinx_sitemap)
  with circinus-tailored html_baseurl https://docs.vyos.io/en/1.5/ and
  curated docs/_html_extra/llms.txt for 1.5.x.
- Updates docs/_html_extra/robots.txt with AI crawler Allow rules and
  the /en/1.5/sitemap.xml reference.
- Drops configuration/service/suricata from the swap set (no matching
  RST sibling on circinus; suricata is a current-only feature).

Supersedes:
- #1885 (broken md-*.md converter output)
- #1869 (LLM doc adaptation backport, bundled here)
- #1875 (llms.txt circinus, bundled here)

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