Update AI architecture docs

What does this MR do and why?

Follow up to !131628 (merged)

• Updated architecture diagram to better reflect status quo
• Create SSoT for diagram PNG (it existed twice in binary form)
• Added section to new SM/Dedicated docs
• Updated Code Suggestions table (where do things happen now vs future)
• Some smaller cleanup

Preview: https://main-ee-131772.docs.gitlab-review.app/ee/development/ai_architecture.html

TODO:

• Update https://docs.google.com/drawings/d/1PYl5Q5oWHnQAuxM-Jcw0C3eYoGw8a9w8atFpoLhhEas/edit with the new version of the diagram once approved

Architecture diagram updates

• Removed references to "GitLab Plus" and Cloud Connector. This has caused quite some confusion because readers assumed there was (or should be) a "Cloud Connector service". We should strictly separate documentation for the current state of GitLab from documentation/discussions around how to evolve it. We can use issues/epics and the Architecture Workflow to do that.
• Updated the IDE layer to collapse vendors into a single box, and included the web fontend to highlight that there are AI use cases serviced outside of IDEs (chat, MR summaries etc)
• Updated the AI gateway box to be more faithful to what is currently implemented. Moved Access Layer into the AI gateway since that's where it currently lives. Again, this is not to say we won't have some sort of access abstraction fronting the AI gateway in the future, but we don't now.
• Clarified the role of CustomersDot and how it interacts with the AI gateway and self-managed instances. Separated it visually from GitLab.com, which does not interact with it.

Before

After

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #422369 (closed)