Add details on sparsity

[gdalle]

Docs

• Add section on sparsity to the tutorial, documenting the unexported objects that make it work

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.27%. Comparing base (7d3b258) to head (0a3bc4a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #237   +/-   ##
=======================================
  Coverage   96.27%   96.27%           
=======================================
  Files          74       74           
  Lines        3924     3924           
=======================================
  Hits         3778     3778           
  Misses        146      146           

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