Write more thorough documentation

* make a full sphinx documentation for the project with
* proper manual on how to use it (and why not generate a manpage?)
* documentation of internals
* upload of all of it on readthedocs

The issue names no files, tests, or existing documentation entry points. Start by locating the Python CLI entry point and current documentation, then scope the Sphinx manual, internals reference, manpage decision, and Read the Docs publication. Done means the documentation is complete and available on Read the Docs.

Written by the indexing model from the issue text.