Sphinx documentation

[jacobhinkle]

Is your feature request related to a problem? Please describe.
We currently have https://lagomorph.readthedocs.io/en/latest/ set up but there is no documentation yet.

Describe the solution you'd like
Add simple sphinx docs.

Describe alternatives you've considered
NA

Additional context
The datawork documentation works pretty well I think: https://datawork.readthedocs.io/en/latest/
However, for lagomorph it would be good to group the API documentation separate from an introduction and guides, and perhaps even having an LDDMM overview section. Documentation building should also be part of the CI pipeline.