[Docs] Document how to build and deploy tunnelbroker

Summary:
After testing the deployment, document how it was able to be built
and ran.

Part of https://linear.app/comm/issue/ENG-4104

Test Plan: N/A. Documentation

Reviewers: varun, ashoat, bartek

Reviewed By: ashoat, bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8312