Better backend docs

[gdalle]

• Add functions to check support for mutation and second-order differentiation
• Use them to generate tables of backend functionalities, including backend combinations for hessians

[gdalle]

Output preview (the table is scrollable):

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.47368% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 90.31%. Comparing base (6a771fb) to head (2a79f94).

Files	Patch %	Lines
src/backends.jl	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   90.26%   90.31%   +0.05%     
==========================================
  Files          48       48              
  Lines        1653     1663      +10     
==========================================
+ Hits         1492     1502      +10     
  Misses        161      161              

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