CI: Add docbook-cs linting

[jordikroon]

As announced here: https://news-web.php.net/php.doc/969388652
See for test implementation: #5510

The sniffer includes:

This also makes check-whitespaces.xml obsolete, as the sniffer will check for that as well.

The sniffer also covers #5445

• <simpara> should be used instead of <para>
• Exception or error class names wrapped in <classname> that should use <exceptionname> instead.
• Whitespace checks (.github/workflows/check-whitespace.yml became obsolete)
• Ensuring that when an element has both xml and xmlns attributes or xmlns variants, xml appears first (covers Add CI warning for xml:id before xmlns attribute order #5445)

[TimWolla]

Some remarks for the workflow definition. Didn't look at the tool itself, but the premise sounds good to me.

[TimWolla]

Some more quick GHA comments. Will need to think about the git / checkout usage more at a later point.

[jordikroon]

I am planning to merge Docbook-cs in a few days as all feedback has been addressed. Please let me know if there are comments, objections or concerns regarding this new tool. So to say any feedback is welcome.