Add howto doc for setting up Gitlab.com OAuth

What does this Merge Request do?

Adds instructions on how to set up Gitlab.Com OAuth with your local gitlab instance

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.

I don't think the above are relevant 🤔