Add broken Xref checker

[settermjd]

This PR adds a custom Antora generator script that can find and report broken Xrefs in the documentation and updates the docs to show how to use it.

[mmattel]

site.yml --> site.prod.yml

[settermjd]

That would have to happen later, as that file's not part of this branch. The docs can be updated later.

[mmattel]

I would reference to the local ui-bundle.zip not the one from docs/releases

[settermjd]

I believe the release artifact is the right one, for the time being, until the build pipeline is finalised.

[mmattel]

sorry disagree. we currently do have regular updates in docs-ui and as long the automated process is not in place, any reference to an old bundle will counteract the updates made. just my 2c

[mmattel]

First
I used that branch to test, but I get the following error

error: Generator not found or failed to load.

with option --stack-trace:
Error: Cannot find module '@antora/content-aggregator'

Second
I created #205 (Have an own page for a broken link checker)
Why not adding it there ? Would make more sense to me as the other once can also check external links.

[settermjd]

That error shouldn't appear when the Antora packages are installed locally. As #216 is merged to master, and rebased now onto this PR, it should be fine to merge this now. I'll update the README to cover the inclusion of package.json and package-lock.json in a separate PR.