[docs] update docs with instructions for running identity service

Summary:

• added new docs for services setup and services workflows
• added instructions for setting up docker and running the identity service
• fixed a broken link and added xcode installation instructions

Test Plan: made sure links all work

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7261