Eliminate duplicate docs across site and repos

(AI from gRPC Docs Working Group 03/30)

This issue concerns keeping out docs [DRY](https://en.wikipedia.org/wiki/Don%27t_repeat_yourself) across grpc.io content and repo content, be it https://github.com/grpc/grpc or any of the language specific sites like https://github.com/grpc/grpc-java.

For example, https://github.com/grpc/grpc/blob/master/examples/cpp/cpptutorial.md is an outdated version of https://grpc.io/docs/tutorials/basic/cpp.

Also in scope of this issue is the cleanup of code repo README files so that they, for example, include better references into the appropriate language pages on grpc.io.

- [x] C++ Hello World: refer reader to Quick Start grpc/grpc#22636
- [x] C++ examples README cleanup grpc/grpc#22635
- [x] Removing obsolete C++ tutorial content grpc/grpc#22633
- [x] Go `examples/README` cleanup https://github.com/grpc/grpc-go/pull/3738
- [ ] Migrate docs from grpc-web repo to here #271