Add support for cmake snippets in documentation

[nmellado]

See details and link here: https://github.com/STORM-IRIT/Radium-Engine/pull/591#discussion_r459696611

[MathiasPaulin]

For quicker access, here is the link : http://jesnault.fr/website/document-your-cmake-code-within-doxygen/

[stale[bot]]

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[nmellado]

Another option is to use the \dontinclude command (https://doxygen.nl/manual/commands.html#cmddontinclude)

The main limitation I have observed is that files with the same name will not be parsed (only the first one is retained, depending on the ordering of doxygen variable EXAMPLE_PATH). As such, I don't know how to document any other file CMakeLists.txt than the main one (if the project root is the first one included), but this will work for all the other cmake files we have in the project, e.g. RadiumSetupFunctions.cmake.