Improve documentation

[zblz]

Currently the docs are comprised of a tutorial rendered from a notebook, and an API reference built form the python docstrings. We should expand the docs to include:

• An introduction to lens.
• A motivation of the choice of precomputing the summary vs computing it on-the-fly. Advantages of using dask?
• Tutorial for computing a summary and using the Summary and Explorer classes.
• Contribution guidelines.
• Structure API docs in a more clear way.