Moved documentation to Sphinx

This pull request addresses issue #398: moving the documentation to Sphinx.

I have combined and converted the documentation from the github wiki, the trac site, and the README to Sphinx in the `docs` folder. In addition, I checked the formatting of each page in the new Sphinx documentation to make sure each one displayed properly after being built.

Since I am not an experienced user or developer of WYMeditor itself, I did not modify any of the content on the documentation pages beyond fixing typographical and grammatical errors, but instead, I combined and formatted the already existing documentation found in the github wiki, trac site, and README into one cohesive Sphinx structure. The overall structure I used for the documentation was based on how the documentation was presented on the github wiki. This structure may not be ideal, but it can be easily adjusted at a later time by modifying the underlying Sphinx structure.

Because I did not modify the actual content of the existing documentation, it should be noted that the areas in the existing documentation that had missing or incomplete descriptions still have missing or incomplete descriptions in the Sphinx documentation.

Finally, I tested building the documentation on [Read the Docs](https://readthedocs.org/) from my fork of WYMeditor, and it appears to have built successfully. You can view the test [here](https://wymeditor-doctest.readthedocs.org/en/latest/).