[Docs] Document how to deploy identity service
Summary:
Document how to deploy identity service, so that if another member
needs to address the deployment, they don't need to reverse engineer the actions.
https://linear.app/comm/issue/ENG-4420
Depends on D8580
Test Plan: N/A. documentation
Reviewers: varun, ashoat
Reviewed By: varun
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D8581