[documentation] Add a section about classification of SIF files

[amontoison]

• Also added a function select_sif_problems.

[codecov]

Codecov Report

Attention: Patch coverage is 18.18182% with 18 lines in your changes missing coverage. Please review.

Project coverage is 74.20%. Comparing base (f2ad6a4) to head (373583a).
Report is 96 commits behind head on main.

Files with missing lines	Patch %	Lines
src/classification.jl	18.18%	18 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #411       +/-   ##
===========================================
- Coverage   89.11%   74.20%   -14.91%     
===========================================
  Files           5        7        +2     
  Lines         790     1640      +850     
===========================================
+ Hits          704     1217      +513     
- Misses         86      423      +337     

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