Create initial AI setup page

What does this merge request do and why?

Create a single source of truth for setting up AI services for local development at GitLab.

• Establishes which AI services are supported in GDK and which need manual setup.
• Provides a high-level architecture diagram showing how AI components interact in a local development environment.

The primary audience is GitLab developers who need to set up and develop AI features locally using GDK.

See #3041

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 MR references an issue describing the change.
• 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.