doc: doxygen: Enable Doxygen autobrief

Enable Doxygen AUTOBRIEF feature for both Javadoc and Qt style
documentation blocks. This treats the first sentence of a documentation
block as a brief description even when no @brief tag is present. This makes
the overview sections of the generated documentation much more useful.

See https://www.doxygen.nl/manual/config.html#cfg_javadoc_autobrief and
https://www.doxygen.nl/manual/config.html#cfg_qt_autobrief



Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>