docs: gitlab: add an explanation for required access token scope

Created by: slimsag

A customer asked me why we need an api scoped access token, since according to GitLab's docs this:

Grants complete read/write access to the API, including all groups and projects, the container registry, and the package registry.

I did a deep dive to find the answers here, and this PR adds a section to our docs that explains it in detail while still remaining future-proof (so hopefully it doesn't become too out of date in the distant future).