Resolve "Run gdk doctor in CI"

What does this Merge Request do?

This MR adds a new CI job called verify:doctor which runs gdk doctor 🙂

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.

Closes #865 (closed)