Switches from Sphinx to MkDocs, since it:
* supports Markdown natively without requiring hacks like `eval_rst`
* validates inline links, ensuring that they are not broken
* has a more pleasant live-reloading dev-server
* supports the nicer looking mkdocs-material theme
* is a third of the size of Sphinx (including deps)
The theme change is now possible since Read the Docs have just started
supporting use of custom themes (previously they would override the theme
and use `readthedocs` theme regardless).
Ed Morley