(nestjs) Add note about using express.raw() before fedifyMiddleware

- Updated integration.md and packages/nestjs/README.md with a note explaining
  why `express.raw()` must be applied before `fedifyMiddleware`.
- Clarified that Fedify requires the raw request body to perform HTTP signature
  verification for ActivityPub requests, which can break if the default body
  parser consumes or transforms the request body first.