Write documentation for setting up and running a production instance of h

A number of parties have come to us looking to do small/medium-scale deployments of h, eg. within an organization. Our documentation at https://h.readthedocs.io currently only explains how to run h locally in development, it doesn't say anything about how to do a production deployment.

While different sizes of deployments are going to have some differences with respect to how a deployment works, a lot of what needs to happen (eg. building and running Docker containers, supplying configuration via env vars, setting up Postgres/ES/RabbitMQ) will be the same regardless of the infrastructure.

This documentation should cover topics such as:

- Building and running the Docker image of h (or perhaps using our existing Docker image)
- Setting up and connecting h to the required services
- Setting up the database and ES indexes
- Handling authentication
- Updating to a newer version of h
- Supplying configuration via environment variables
- What needs to be backed up and what doesn't
... and no doubt a bunch of other things I haven't mentioned here

There are other issues to think about as well, such as how we manage versions designed to be used by third parties.

Start with the existing development documentation at h.readthedocs.io and review the Docker deployment approach and required Postgres, Elasticsearch, and RabbitMQ services. Done means documenting production setup, configuration through environment variables, authentication, database and index setup, upgrades, and backup guidance.

Written by the indexing model from the issue text.