CI: add lychee link check on built docs

[InessaPawson]

Adds a links workflow that runs https://github.com/lycheeverse/lychee against the rendered MkDocs output. Closes #473.

Checking the built site instead of the markdown sources means links that only exist after rendering (magiclink shorthand, autorefs, mkdocstrings) are covered, which answers the question in #473 (comment).

site/404.html is excluded because MkDocs gives it root-relative /numtype/... links that only resolve on the deployed site.

Tested locally with uv run --only-group=docs mkdocs build followed by lychee:

🔍 273 Total (in 1s 582ms) 🔗 88 Unique ✅ 266 OK 🚫 0 Errors 👻 1 Excluded ⛔ 6 Unsupported 🔀 4 Redirects

The scheduled run won't fire on a fork PR, so this needs a workflow_dispatch after merge to see it in action.

[jorenham]

Thanks!