Add gitlab-docs link on howto documentation

What does this Merge Request do?

Make the documentation a little bit easier to follow by adding a link to gitlab-docs repository.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.