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.
There is `sphinxcontrib.httpdomain` which we could use instead, like this: https://raw.githubusercontent.com/praekelt/go-contacts-api/develop/docs/go-contacts-api.rst , But I think this will look good just as is. Just maybe something to look into in the future.