Draft: AI Context Policy blueprint

What does this MR do and why?

This is the draft for the architecture blueprint for the "AI Context Policy" management.

The very first draft iteration in the context of Language Server: Draft: feat: introduced the AI context policy (gitlab-org/editor-extensions/gitlab-lsp!371 - closed)

View rendered markdown at https://gitlab.com/gitlab-org/gitlab/-/blob/dmishunov/ai-context-policy-blueprint/doc/architecture/blueprints/ai_context_policy/index.md

Where does this proposal come from?

The work in the blueprint to that or another degree is related to:

• Slash commands in GitLab Duo Chat (&13947) (groupduo chat)
• Advanced Code Context Resolver Architecture - (... (gitlab-org/editor-extensions&55 - closed) (groupeditor extensions )
• RAG MVP for Code Suggestions - External Documen... (&13924) (groupcustom models )