Add support for Docs

We could easily migrate the [Scientific Python template](https://github.com/scientific-python/cookie) support for ReadTheDocs over to this repo, but I've so far preferred to keep things as minimal as possible. Probably worth doing this and putting it as a question, e.g. 'Do you want documentation support?'

Side note -- copier being able to update projects *after* they've been generated means that you can always re-add docs support after the fact!