Improve structure of Briar Headless API documentation
Currently, the documentation for the API is directly written into [its readme file](https://code.briarproject.org/briar/briar/blob/release-1.1.7/briar-headless/README.md). That's fine for the beginning, [I was even able to build a first prototype with it](https://nico.dorfbrunnen.eu/posts/2019/briar-first-demo/).
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 https://code.briarproject.org/briar/briar/issues/1577.
**Update:** I agree with Torsten that it's best to generate the documentation from code. That's what I do with [`briar_wrapper`](https://code.briarproject.org/briar/python-briar-wrapper), too, at https://wrapper.docs.briarproject.org/ and the API's docs can be hosted at https://api.docs.briarproject.org/, imho.
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