Add documentation for the API

[toxdes]

API isn't documented, and changes every now and then, because it's still about to settle down.

A good documentation of the API would help, although I'm not sure if I should be writing it manually or just use some sort of doc-generator.

[toxdes]

There's a few options.

1. Use swagger and jsdoc to document the methods inline.
2. Use prisma-doc-generator, but this will generate docs for the prisma schema, and not for the API routes.
3. Use custom markdown file, and use a static site generator like docasaurus.