Add support for other documentation projects and dynamic builds

What does this Merge Request do and why?

Following up from !1896 (merged).

Adds ability to turn on and off the "extra documentation projects".

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.