Documentation changes to account for gdk ai setup command

What does this merge request do and why?

This change adds documentation changes for the GDK command to set up AI Gateway in GDK. The GDK command MR can be found here: !3822

The GDK command aims to move the setup instructions from this doc into an interactive one line command to help set up the local environment for AI development. This change also edits and cleans up documentation based off of the use of this rake task.

Related:

• !3822
• !3783 (closed)
• gitlab!156970

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

To try locally, from the GDK root directory, run bundle exec rake gitlab-ai-gateway:setup

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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 an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.