src-expose: document usage with docker compose and kubernetes

Created by: keegancsmith

Write two short sections at the end of the https://docs.sourcegraph.com/admin/external_service/non-git docs titled Using src-expose with a Docker Composes deployment and Using src-expose with a Kubernetes cluster deployment.

Those sections could be short, or link to Ryan’s repo, but should cover the tradeoffs of different ways of doing it (e.g. if they just SSH in and run it alongside Sourcegraph, what happens if the system restarts? Etc.)

Docker and Kubernetes examples for using src-expose at https://github.com/sourcegraph/src-expose-demo/

Note: avoiding modifying docker compose file is preferred.

Context: https://sourcegraph.slack.com/archives/CJX299FGE/p1586472552293000