Move documentation and examples to external docs site

`label-maker` has grown a lot in terms of options and examples, and all the new content has made it difficult to parse the README. We should move the docs and examples to one of the autogenerated docs platforms so it's better organized.

We first need to decide on a platform. Python offers some [guidance here](https://wiki.python.org/moin/DocumentationTools) and the [ReadTheDocs](https://docs.readthedocs.io/en/latest/getting_started.html) platform is also pretty popular. Any other good options?