Add Backend Integrations doc and links Introduce Backend-Integrations.md documenting the default-off slslite:request channel for trusted Paper NPC, menu, and backend plugins. Explains Velocity player derivation, authorization by server/blueprint, action allowlists, and request safety (size bounds, rate limits, validation, deduplication). Notes that general forwarders are unsupported and links to the canonical Backend Messaging guide and a Paper backend sender example. Also add links to the new doc from Home.md and _Sidebar.md.