Refactoring of the BandMathX documentation, and fix OpticalCalibration documentation

Summary

This merge request replaces the AboutBandMathX C++ section by a recipe: Operations on images with BandMath and BandMathX

The BandMath part of the ImageProcessing recipe has been moved to the new recipe.

This merge request also fix a bug in the OpticalCalibration application documentation (a newline was needed before a bulleted list). See issue #1956 (closed).

Rationale

See issue #1904 (closed)

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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Check before merging:

• All discussions are resolved
• At least 2 votes from core developers, no vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement
• Optionally, run git diff develop... -U0 --no-color | clang-format-diff.py -p1 -i on latest changes and commit