Reorganize rpc API doc and add introduction (#30491)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30491
Our RPC API docs presents the APIs well but misses a general
introduction to the APIs. Readers might be a little lost the first
time landing this page. This commits reorganizes the APIs into
four components from user's perspective, RPC, RRef, dist autograd,
and dist optimizer. It also adds an intro to each and briefly
discribes why we provide those.
Test Plan: Imported from OSS
Differential Revision: D18723294
Pulled By: mrshenli
fbshipit-source-id: 4aced4ab537b070aa780aaaf9724659fd47cb3cb
