Doc: add vertex AI search setup

What does this merge request do and why?

Inform devs to also set up AIGW_VERTEX_SEARCH__PROJECT for documentation search to work.

How to set up and validate locally

N/A

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.