[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