Allow user to generate open-amp API Doxygen documentation in a simple way

[arnopo]

The current status is that we have 2 Doxygen configuration file:

• https://github.com/OpenAMP/open-amp/blob/main/Doxyfile : use to generate documentation using Doxygen application
• https://github.com/OpenAMP/open-amp/blob/main/doc/Doxyfile.in: used to generate the OpenAMP documentation and optionally generate documentation during build command ( WITH_DOC cmake option)

We need to provide a simple way for contributors to generate the Doxygen API documentation, without duplicating the Doxygen configuration files.

The proposition is:

• remove the Doxyfile in the root of the project ( merge some option in the doc/Doxygen.in)
• add Doxygen installation instructions ( README.md and or a requirements.txt file)
• add DOC_ONLY cmake option to be able to build Doxygen documentation in a simple way

cmake -Bbuild -DDOC_ONLY=on
make -C build doc 

[wmamills]

And test it on Linux Windows and macOS.
I will handle that part if you do the cmake stuff.
(I am a cmake beginner)

[github-actions]

This issue has been marked as a stale issue because it has been open (more than) 45 days with no activity.