Docs: Resolve "No rule to make target" error

What does this Merge Request do and why?

I noticed this error whenever running make gitlab-docs-check:

make: *** No rule to make target `gitlab-docs/rm-nanoc.yaml', needed by `gitlab-docs/nanoc.yaml'.  Stop.

This addition of the rule is a 1st stab at solving it.

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.