docs(llms.txt): point links at .md sources, not .html

Per the [llms.txt spec](https://llmstxt.org/), `/llms.txt` links should
point at markdown files (the spec example: `[API docs](https://example.com/api.md)`).
We were using `.html` URLs, which still work but force LLM consumers
through HTML parsing for content that's already available as raw MyST
at the same path (the `.md` is copied to HTML root by
`_copy_md_sources`).

Pivot the curated llms.txt template to `.md` URLs across 22 of 23
entries. The single exception is `cli.html` — sagitta's `docs/cli.rst`
hasn't been migrated to MyST (rst-to-myst chokes on the heading
hierarchy), so no `cli.md` is served at the HTML root. A Jinja
comment in the template documents the exception and references the
follow-up cli.rst migration as a separate PR.

Verified locally: every linked `.md` path returns 200 in production.

Same pivot applied symmetrically across [#1874](https://github.com/vyos/vyos-documentation/pull/1874) (current),
[#1876](https://github.com/vyos/vyos-documentation/pull/1876) (sagitta), and [#1903](https://github.com/vyos/vyos-documentation/pull/1903) (circinus).

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