Migrate documentation from grpc doc folder to the grpc/grpc.io repository

Currently, I see two main sources of documentation for the grpc.io:

- There is this website, generated by hugo.
- There is the doc folder in the grpc repository: https://github.com/grpc/grpc/tree/master/doc

I think it would be a good idea to have a centralized place for the documentation so that contributors know where to contribute new content.

I would suggest to migrate it here as it will have a clean URL that will make it easier to find through a web search.