docs: give the self-hosted model lane its own page The lane was 27 lines under Local Development, which mis-scoped it: it applies to any deployment someone runs, not just a laptop stack. Self-Hosted AI Models now carries both lanes in one place (MCP clients bring their own model; operators call what the deployment configures), the full settings table, which endpoints work, the twelve-question results, and what to check before pointing it at real books. Local Development keeps a pointer under the same anchor; the AI Operators and Architecture cross-references now land on the new page. Corrects one claim while moving it: mapping all three tiers does not free a stack from AWS credentials outside dev, since the platform checks them at startup for storage and the graph registries regardless.