Improve structure of Briar Headless API documentation

Currently, the documentation for the API is directly written into its readme file. That's fine for the beginning, I was even able to build a first prototype with it.

However, I suggest to improve the structure of the documentation by moving it into a separate repository. My suggestion was to create a Hugo page with a docs theme (or some other static site generator) at docs.briarproject.org.

@grote's response to this was (loosely translated by me): "Best to make the documentation directly into the code with swagger"

My response to this now: OK, we can do this, but I don't know how much sense it makes to put a lot of basic explanations into code. E.g. all the valuable information collected in #1577 (closed).

Update: I agree with Torsten that it's best to generate the documentation from code. That's what I do with briar_wrapper, too, at https://wrapper.docs.briarproject.org/ and the API's docs can be hosted at https://api.docs.briarproject.org/, imho.