Update Documentation

Based on conversations regarding the recent changes, and user feedback, it would be good to do a review of our current documentation, finding areas for improvement. Examples include:
- [x] Add a "how to build a catalog"
- [ ] What is intake-esm doing with my data?
- [ ] What is a dictionary of datasets/collection?
- [x] How do I add a derived variable?
- [ ] How to disable aggregations in `to_dataset_dict()`
- [x] How to modify `groupby_attrs` at runtime (#522 )
- [x] How to save a subsetted catalog (#523 )
- [ ] Document the `skip_on_error` flag in `to_dataset_dict()`
- [x] How to load a single key via the `cat[key].to_dask()` (#522 )
- [x] How to use the `cat.keys_info()` to retrieve information about keys (#522 )
- [ ] Guide on how to debug when things go wrong...
- [x] https://github.com/intake/intake-esm/issues/404
- [x] https://github.com/intake/intake-esm/pull/412: [furo theme](https://github.com/pradyunsg/furo)

Based on this [documentation system](https://divio-documentation-system.readthedocs-hosted.com/), we should cover
* Tutorials
* How to Guides
* Explanation
* Reference