Update backend documentation

[adrhill]

• summarizes all check_* functions in one table
• adds examples on how to use ADTypes, e.g. to define forward- and reverse-mode in Enzyme

Preview: Backends.pdf

[gdalle]

This is amazing! Maybe replace "mutation" with "two arguments" to keep terminology coherent throughout?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.05%. Comparing base (b85ea79) to head (377c1d7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
- Coverage   96.40%   96.05%   -0.35%     
==========================================
  Files          65       69       +4     
  Lines        3558     3779     +221     
==========================================
+ Hits         3430     3630     +200     
- Misses        128      149      +21     

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