[Docs] Document keyserver deployment

Summary:
While reviewing D8538, I had difficulty with deploying keyserver
as a docker image and pointing it to production identity service because
it required some additional steps to configure this properly.

https://linear.app/comm/issue/ENG-4416

Test Plan: N/A, documentation

Reviewers: jon, ashoat

Reviewed By: jon, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8577