GHA: docs - merge link checking and build/publish

[peteski22]

What's changing

Now that we can skip running steps in the docs GHAs we can merge the link checker with the docs build+publish again.

Link checking will run regardless of whether /docs folder changes are detected, if it fails the Docs - Build and publish job will fail.

How to test it

https://github.com/mozilla-ai/lumigator/actions/runs/11921009359/job/33224218774?pr=397#step:4:1

Additional notes for reviewers

See: https://github.com/mozilla-ai/lumigator/pull/394

I already...

• [ ] added some tests for any new functionality
• [ ] updated the documentation
• [ ] checked if a (backend) DB migration step was required and included it if required

[peteski22]

~Closing as link checking happens on all .md files not just within /docs. Another PR to follow to resolve this.~

Re-opened as it's fine to do the check for links as part of the docs job, it should always run (so no conditional check needed).