Update documentation for QGIS, Python and multiwriter

Summary

Update Cookbook documentation for :

• Python : document conda (linux) and bindings re-compilation from binary packages
• QGIS : Copy documentation for parameters from the plugin ReadMe.txt (the plugin from otb gitlab should not be used with QGIS 3.8, as it is now in QGIS core) and document compatibility issue between otb and qgis versions (otb 7.1 is not compatible with QGIS <= 3.12).
• Multiwriter : solve a bug with an unitialized boolean (IsMultiWritingEnabled) before DoExecute(). A consequence that documentation for multiwriting was not correctly generated for applications (a paragraph about multi writing is automatically generated for applications that support it, but before this fix it was generated at random for applications that don't support it)

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