Update documentation - testing/development

 * add sphinx_rtd_theme theme to dev requirements
 * change theme to sphinx_rtd_theme
 * fix section headers - use chapters where needed
 * refresh documentation where needed
 * add docs on development and testing

Additionally remove incorrect comment from code.
Hold on, can we instead refer to the actual file `tox.ini` instead of copying the contents to the documentation? Isn't the code itself more likely to be accurate? I refer to the file you're adding in #93.