Followup to #473, which made minimal changes -- it _only_ relocated the Android and Web pages without changing them. This PR moves the Android pages (which are Java specific) to a sublevel, like this:
- /docs/
- platforms/
- android/
- **java/{quick-start, basics,...}**
- kotlin/quick-start
- dart or flutter ...
- web/{quick-start, basics, ...}
Context: https://github.com/grpc/grpc.io/pull/473#issuecomment-714048028