Add processing documentation

[sdebionne]

Description

Add documentation for

• sharpening and distance_transform
• adaptative histogram equalization (ahe)
• convolution, morphology and thresholding

Move histogram section under image processing.

References

Closes #804

Tasklist

• Ensure all CI builds pass
• Review and approve

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.85%. Comparing base (c889bcb) to head (3cf4ace).
⚠️ Report is 27 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #805   +/-   ##
========================================
  Coverage    82.85%   82.85%           
========================================
  Files          119      119           
  Lines         5634     5634           
========================================
  Hits          4668     4668           
  Misses         966      966           

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