add link checker to ci

[6543]

add a link checker for the docs into our pipelines.

https://lychee.cli.rs/ should be a good way to go.

this should run as a docs linter ... only for files changed and block
and weekly check the whole docs ... but do not block

[6543]

quick run:

so yes we need it!

[pat-s]

lychee is great. Using it in some projects, fast and reliable.

[anbraten]

Wasn't this error caught by: https://docusaurus.io/docs/api/docusaurus-config#onBrokenLinks

[6543]

Well docusaurus can at most check internal links, we should check all