JWT specifications

[devnote-dev]

Having a detailed specification for JWT token generation and the usage in the different APIs (this is partially documented already via the claims sections). This is primarily for the Wings API but also includes a couple of endpoints from the client API.

[real2two]

I created a PR for Wings API JWT specifications. #14

This PR should not be closed after the PR is committed, because I didn't write JWT specifications for the client API.