Fix CookBook build inside a virtualenv & add "Compiling documentation" subsection

Summary

CMake < 3.13.0 has a bug and so works poorly when calling Python during configure from inside a virtualenv.

• Fix CookBook build inside virtualenv
• Improve python version detection in CookBook build
• Add documentation on how to build documentation

Rationale

Building the cookbook depends on: numpy, sphinx, sphinx_rtd_theme which are not always available on the system.

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