docs: (docker-compose) recommend that users create their own fork

Created by: ggilmore

This PR changes our docs to recommend that users create a fork to store their customizations to https://github.com/sourcegraph/deploy-sourcegraph-docker/blob/master/docker-compose/docker-compose.yaml. Users will need to customize the yaml to change resource requirements, replica counts, customize environment variables, etc.

Our recommended merging strategy is the same one that we recommend in deploy-sourcegraph: https://github.com/sourcegraph/deploy-sourcegraph/blob/master/docs/configure.md#fork-this-repository