CI: Check html links for doc

[hdiethelm]

Check HTML links in docs in CI

There are a few broken links, so continue-on-error is set to true for now.

This is just one tool of many. I tried:

• https://github.com/wjdp/htmltest
  • In this PR
  • Really fast
  • Looks not maintained to much lately
• https://github.com/gjtorikian/html-proofer
  • Takes minutes instead of seconds
  • Checks also other HTML issues
• https://github.com/w3c/link-checker
  • The output is quite verbose
  • Could be an alternative
  • No up to date binary / docker image, we would have to build it from source
• https://github.com/JustinBeckwith/linkinator
  • I did not manage to get usable output

[grandixximo]

LGTM. htmltest in a container works well and keeps doc builds free of new dev dependencies.

One note for later: we already have an in-tree link checker (docs/src/checkref) built on w3c-linkchecker, but it has been a silent no-op for a while since recent w3c-linkchecker refuses file:// URIs, so it validates nothing. This CI check is a real improvement over that.

If we ever want something more actively maintained than htmltest, lychee (Rust, async) is excellent. I'd keep it CI-only via Docker though: there is no Debian apt package for it, so it is not a good fit for the local make checkref target where we rely on apt-installable deps.