Add documentation (README) for Ray service

Description

Currently, the Ray service is missing basic documentation. A README should be added which includes the following:

• A description of what the Ray service is and how it is used in NDIF.
• Directory structure + explanation of main files.
• List all the main classes (Controller, Cluster, Node, etc) and give a brief description of what they are.
• A brief description of the main library dependencies and what they are used for (and remove any that are no longer being used from environment.yml)
• A list of any relevant environment variables and what they are used for (checkcompose/dev/docker-compose.yml, compose/dev/.env, and the source code).
• Add a section for spinning up the Ray service from the command line (I can help with providing content). Can mention that it can be spun up but won't work without the API.

And anything else you think would be useful to add.

Even if you don't have answers to most of these questions, feel free to kickstart a PR - one of the developers would be happy to work with you on it.