docs: render llms.txt at build time (circinus)

Parity with [#1874](https://github.com/vyos/vyos-documentation/pull/1874) (current) and [#1876](https://github.com/vyos/vyos-documentation/pull/1876)
(sagitta). Replaces the static `docs/_html_extra/llms.txt` with a Jinja
template (`docs/_templates/llms.txt.j2`) rendered at build time via a
`build-finished` hook in `conf.py`. URLs and the version line are
interpolated from `html_baseurl` and `release`, so the rendered file
always matches what circinus actually serves.

Hook behavior matches `current`/`sagitta` exactly, including the two
review-cycle hardenings landed on those PRs:

- Skips on builders other than `html` and `readthedocs` — the curated
  template encodes `.html` URLs which don't match `dirhtml` output.
- `RuntimeError` if `html_baseurl` is empty rather than silently
  rendering broken root-relative URLs.
- Jinja `Environment(undefined=StrictUndefined)` so a typo in
  `llms.txt.j2` raises at build time instead of writing a half-blank
  `llms.txt`.

The closed PR [#1875](https://github.com/vyos/vyos-documentation/pull/1875) was the previous attempt; this re-lands the change after the
post-flip rebase situation cleared.

Verified locally: hook tests (happy-path render, fail-fast on
`html_baseurl=None`/`''`, `StrictUndefined` catches `{{ relase }}`
typo, skips `dirhtml`, skips on build exception) pass against this
worktree.

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