doc: CMake: Locate Python executable before processing modules

The module processing logic in Zephyr requires Python, since it uses a
Python script to locate and process the modules. It is therefore
required to find the Python executable using CMake before actually
including the CMake script that processes the modules. In order to
prevent this from happening again, move the inclusion of version.cmake
as well in case that script chooses to use Python in the future.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>