worker: Add docs
Created by: efritz
Add docs to explain how to deploy and configure the new worker service. Fixes https://github.com/sourcegraph/sourcegraph/issues/21759. This PR updates:
- the changelog to call out a new service
- the update docs to call out a new service (k8s, pure docker, and docker-compose)
- an admin doc on what a worker is, what jobs it defines, and how to scale it
cc @sourcegraph/cse for changes that may affect upcoming installations
cc @sourcegraph/distribution for specific advice on upgrade docs (do we need to call out specific instructions to include a new container?)
The last doc is a bit light on details on when to know how to scale because we don't currently have the tooling (only educated guesses on what is taking disproportionate memory or compute in certain circumstances). I think this is something that will clarify itself as we gain more operational experience.