Revise getting started guides and Readme

[willow-ahrens]

These have gone a a bit stale, and don't cover a few key points:

• [ ] Reorganize docs to center high-level interfaces (#621)
• [ ] High-level API fusion
• [ ] Common format conversion and getting started workflows
• [ ] core concepts such as concordant iteration and densification
• [ ] self-contained runnable scripts in the examples

[willow-ahrens]

Additionally, it has been noted that many of the examples aren't self-contained runnable scripts. I think it would be valuable to have those.