Draft: docs: Add linter for the documentation

[kosstennbl]

Description

Add mdl as a linter for new cnf-testsuite documentation Add custom rules for keeping format of the documentation Add lint to git workflows to run mdl. Remove docs directory from .gitignore Fix small issues in documentation for linter to pass.

Issues:

Refs: #1945

[martin-mat]

1. I'm wondering whether it is a good place to store *.rb rules in the doc directory. To me it looks like CI tooling issue and perhaps should be somewhere else, like /utils dir ...?
2. ~How do we handle introduced ruby dependency (although for CI/testing purposes only)?~ I see, handled inside container in gh actions.
3. Can we exclude the doc linters/workflow from triggering the crystal workflow?