doc: doxygen: add version and shorten brief

Render Zephyr version in both, standalone Doxygen build and Sphinx
controlled build. In standalone mode, the package version given by
find_package(Zephyr...) is used, while in Sphinx build the conf.py
version is used.

The project brief has been shortened to make project title more compact
while still containing relevant information.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>