Add a link checker in CI

[maxnoe]

I noticed many broken links in the docs over time.

How about we add a link checker to the CI, e.g. this one: https://github.com/lycheeverse/lychee-action

[rdimaio]

We already have one https://github.com/rucio/documentation/blob/main/.github/workflows/check_external_links.yaml, but it might not be performing correctly if it's not reporting broken links

[rdimaio]

I guess if it's in the context of #619 it'd see that it returns HTTP 200 and doesn't complain

[maxnoe]

It's called "external link check", but here we are talking about internal crosslinks.

[maxnoe]

The linked-to page correctly returns 404:

❯ curl -f https://rucio.github.io/documentation/html/client_api/
curl: (22) The requested URL returned error: 404

[rdimaio]

The linked-to page correctly returns 404:

❯ curl -f https://rucio.github.io/documentation/html/client_api/
curl: (22) The requested URL returned error: 404

Weird, I had checked and was getting a 304 Not Modified and then 200, but maybe I was accidentally checking the wrong page.

Lychee seems to indeed support relative links https://github.com/lycheeverse/lychee - I assume they mean the same thing as the internal crosslinks we're meaning here