Self-Hosted Model Deployment Blueprint

What does this MR do and why?

Blueprint for Self-Hosted Model Deployment

Related MVP Self-Hosted Models MVP - Code Generation with M... (&12972 - closed)

Closes Blueprint for Self-Hosted Model Deployment (#448567 - closed)

See also Architecture for Custom Models (#438849 - closed)

Follow-Up issues

• Introduce a Duo AI Features Menu to Allow Users... (#444216 - closed)
• GitLab Duo License Management for Self-Hosted M... (#455858 - closed)
• Deployment of the AI Gateway using Runway (#455854)