Remove Unicorn mentions from docs

What does this Merge Request do and why?

Removes Unicorn from GDK docs.
This is a no-op change, so I gathered it into a separate MR.
Config changes will go in a separate one (I haven't started it yet).

Related to the main repo Unicorn removal effort: gitlab!61888 (merged)

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.