Improve documentation

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.