Update docs to use cloud.gitlab.com/ai

What does this merge request do and why?

See gitlab-org/gitlab#431554 (closed)

Update docs to use localhost to access the AI gateway.

This replaces the old codesuggestions.* host, which should only be accessed by legacy GitLab releases now.

How to set up and validate locally

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

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.