Migrate documentation to Sphinx.

[hodgestar]

Thanks https://github.com/dag/genshi/ for doing a lot of the nitpicky things 13 years ago.

This PR:

• Converts the Markdown documentation to Sphinx RST.
• Converts the API documentation to Sphinx autodoc.
• Adds the Read the Docs configuration file.
• Uses the sphinx-rtd-theme package to provide the theme.
• Updates the README badge to point to the new documentation builds.

Preview of the documentation available at https://genshi-edgewall.readthedocs.io/en/feature-add-sphinx-docs/