Add some basic documentation

Closes https://github.com/rudigiesler/mathml-to-image-service/issues/10

Some people put a bunch of sphinx generated stuff in their repo, once this is in master I'll check if it's necessary, hopefully not.
Neat, I was looking for some example API documentation.

I'll merge this in the mean time, and then work on improving it.

I will use that `httpdomain` stuff and use `autodoc` to document some of the other stuff.