Add & Improve Documentation

[sigmavirus24]

• Turn on ReadTheDocs
• Ensure Sphinx works well
• Write documentation
• Transfer some over from pyyaml.org

[ssbarnea]

While usually is not a good idea to resurect the dead, I cannot ask myself why current version of pyyaml does not seem to have a Sphinx based documentations. That is a problem for other python projects that use Sphinx to build the docs as they cannot get an objects.inv index for pyyaml.