Add documentation (README) for API service

Description

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

• A description of what the API service is and how it is used in NDIF.
• Directory structure + explanation of main files.
• A list of all the endpoints and their functions.
• 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). Emphasize the env vars which developers would be most likely to care about (port, number of guvicorn workers etc)
• State somewhere that we don't yet support running the API isolated from the other services (i.e. can't just spin it up by itself using docker / compose / command line)

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.