Eliminate duplicate docs across site and repos

[chalin]

(AI from gRPC Docs Working Group 03/30)

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For example, https://github.com/grpc/grpc/blob/master/examples/cpp/cpptutorial.md is an outdated version of https://grpc.io/docs/tutorials/basic/cpp.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

• C++ Hello World: refer reader to Quick Start C++ Hello World: refer reader to Quick Start grpc#22636
• C++ examples README cleanup C++ examples README cleanup grpc#22635
• Removing obsolete C++ tutorial content Removing obsolete C++ tutorial content grpc#22633
• Go examples/README cleanup examples: cleanup README.md grpc-go#3738
• Migrate docs from grpc-web repo to here Migrate docs from grpc-web repo to here #271