docs(conf.py): drop dirhtml from llms.txt builder list, use StrictUndefined

Two follow-up Copilot findings on the curated llms.txt render:

1. Drop `dirhtml` from the builder allow-list. The template hard-codes
   `.html` URLs (`quick-start.html`), which don't exist under `dirhtml`
   output (`quick-start/index.html`). Production publishes via the
   `html`/`readthedocs` builders, so `make dirhtml` would only emit
   a misleading file. Cleaner to skip than to branch URL generation on
   builder type for a path we don't actually ship.

2. Use a Jinja `Environment` with `StrictUndefined` instead of a bare
   `Template`. A typo in `llms.txt.j2` (e.g. `{{ relase }}`) now raises
   at build time instead of silently rendering as an empty string and
   shipping a half-blank `llms.txt`. Also set `keep_trailing_newline`
   so the rendered file's terminating newline is preserved.

Addresses Copilot review feedback on PR #1874.

\xf0\x9f\xa4\x96 Generated by [robots](https://vyos.io)

0 files changed, 0 insertions, 0 deletions