docs: Decouple documentation build from the main

 - Documentation is a standalone CMake project in /docs
 - Exclude TFM_VERSION to a dedicated version.cmake file
 - Move /doxygen into /docs folder
 - Adjust documentation for building documentation :)
 - Suppress warnings of PythonModules mismatch in docs build

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I726dc17d9aa547b8faaf35564e21f25aa9c207a9
8 files changed