Create standard documentation on readthedocs.org

[matejak]

API documentation has a de-facto standard of readthedocs + Python Sphinx. Here is a quickstart guide: https://docs.readthedocs.io/en/latest/intro/getting-started-with-sphinx.html The basic integration is already set up: https://nextcloud-api.readthedocs.io/en/latest/

What remains to be done:

• Set up actual documentation structure.
• Include introduction docs, examples.
• Make sure that the API reference is part of the automatically generated docs.

[matejak]

Done in #6 and #13.