Document API

[Merseleo]

At the moment only the most important (especially RPCs) or potentially ambiguous parts are documented.
To ensure a better overview and simplify familiarization with the project for other developers, however, the entire API should be well documented, i.e.

• the RPCs, in particular the meaning of the parameters and the returns and what exactly the method does (especially if there are edge cases)
• the messages and the meaning of the individual fields
• a general description of the API, i.e. roughly which functionalities it offers for which roles, etc.