Separate tests by backend

[gdalle]

Core

• Fix twoarg_support for SecondOrder

Docs

• Remove extensions page

Tests

• Split tests by backend or backend combination to facilitate independent and parallel backend tests
• Make each test file self-contained with the right imports

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.29%. Comparing base (16aa888) to head (aa7a443).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   96.18%   96.29%   +0.10%     
==========================================
  Files          74       74              
  Lines        3931     3937       +6     
==========================================
+ Hits         3781     3791      +10     
+ Misses        150      146       -4     

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