Unified documentation

The dripline-python package is divided between (1) C++ code with python bindings and (2) pure python code built on top of the bound code. From a user perspective, this appears all as one `dripline` package because the bound code, as a package called `_dripline`, is imported into the `dripline` package.

We want to have a unified documentation interface too. Everything that's available as part of the `dripline` package should be documented together as a single package, rather than having separate documentation for (1) and (2) [above].

Ideally the processes for maintaining the documentation would be as minimal as possible. There will be some rst source files in the `doc` directory, and then there will be all of the docstrings in both (1) and (2). Building of the documentation is done by readthedocs.

Building sphinx documentation out of bound C++ code is a thing -- see this [pybind11 documentation](https://pybind11.readthedocs.io/en/stable/advanced/misc.html#generating-documentation-using-sphinx). Binding the pure python code should be straight-forward Sphinx documentation. Then the trick is how to get it all to work together as one package documentation.