Better docs on the reverse-mode compilation process

[yebai]

The current docs on reverse-mode compilation are sparse (#553). More detailed docs on the design and implementation will help other team members understand the code.

[willtebbutt]

This is underway in #459 . My plan is to handle the maths in this PR (so that people can understand what AD is going), and to add a follow up which takes people through the high-level points of the implementation in a systematic way.