docs: correct documentation against the code and restructure the README

[vishr]

What changed

Accuracy. Every user-facing document was audited against the current code after the environment-only configuration change (#305), and each inaccuracy was fixed. Generated pages change through their generator sources.

• Listener defaults: the native binary binds 127.0.0.1:7520; the image sets FANOUT_ADDR=:7520.
• Chat requires FANOUT_AI_PROVIDER plus that provider's key; unselected providers' values and _FILE forms are ignored.
• Setup, recovery, and OIDC flows; the setup banner port; consumed setup-token status codes.
• MCP authenticates with OAuth, not an API key (llms.txt).
• HTTP routes: ingest-token rotation path, OIDC route registration, SPA fallback scope, error shape.
• Alert variables are means of per-minute rollups, and delta variables are relative percentage changes.
• Ingest batch size is a group-commit target; the in-flight budget admits a lone oversized request.
• Collector example gains receivers; shell examples export their variables.
• Testing docs describe the existing axe scans, the dashboard latency ceiling, and CI artifact uploads.
• Development image tags are GHCR-only.

README. Restructured for a first-time visitor (373 to 239 lines): a System overview screenshot in light and dark, what Fanout includes and its preview maturity, a quick start covering Docker, the native installer, telemetry, chat, and MCP, then comparison, architecture, and configuration summaries. The unofficial throughput figure is removed. Ingest internals moved to the storage model page, DuckDB sizing to the configuration reference, and dotenv, generated-asset, and hook details to CONTRIBUTING.

Verification

• just check
• just test-race when auth, API, ingest, query, MCP, or agent paths changed (documentation and comments only)
• User-facing behavior and configuration docs are current
• No credentials, private telemetry, host details, or enterprise-only source are included
• API, migration, ingest, MCP/AG-UI, or release-contract changes are called out (none)
• just docs-generate-check and offline link check pass