Improve User Guide

[CPBridge]

Major update to the user guide:

• The old user guide has now been moved to a "Quick Start" guide. I improved the SR section as it was out of date. I also added an ANN section.
• I added a "General Concepts" section, so far the only page there is "coding", but in the future I will likely add one on coordinates, and possibly other things.
• I have added new pages for each individual IOD with much more complete explanations. The following are finished (the remainder are placeholders for now):
  • SR
  • SEG
  • ANN

This branch has been going since 2021... It's way way past time to get this merged. Since this is purely doc changes and you are busy @hackermd , thoughts on just merging so that readthedocs gets updated and then you can review and tweak later?

[hackermd]

Very nice!