Improve how we document the `gitlabVersion` field

The gitlabVersion field can be dangerous, and should only be used for development or via guidance from Support.

We should do a better job of warning people about this:

1. Warning in NOTES.txt
2. Remove from values.yaml
3. Re-locate documentation to be only within doc/development (edited)

Also:

• update all links to point to docs.gitlab.com/charts/* vs doc/charts/xyz
• Increase the visibility / readability of the top paragraphs.