Support Nanoc data sources

What does this Merge Request do and why?

The GDK accompaniment to gitlab-docs!1742 (merged).

Supports:

• data_sources instead of symlinks.
• Running nanoc with live feature if all docs projects are enabled.
• Running nanoc with view featured if not all docs projects are enabled.

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.