Set up doctests #19
Description
@ben-hampson found a doc bug in #18 that definitely wouldn't have happened if I had doctests, so this bug is a reminder that it wouldn't be a bad idea to set up Sphinx Doctests and then automatically build the documentation. (And maybe get it pushed to readthedocs?)
Currently the docs are in Markdown but I'm happy to switch to RestructuredText if that makes this easier.