PartyLayer/PartyLayer: docs/react-v2-migration.md

# @partylayer/react v1.x to v2.0 migration guide v2.0 adopts TanStack Query v5 as the reactive cache for the data hooks, adds the `@partylayer/react/query` entrypoint, and renames the legacy SDK-layer session getter. The context-based provider and components are unchanged. Most apps need one setup c