doc: enable SEPARATE_MEMBER_PAGES compatibility
doc: enable SEPARATE_MEMBER_PAGES compatibility

Enable Breathe SEPARATE_MEMBER_PAGES compatibilty; this option allows
resolution of a huge amount of references that are currently broken
(around 650 based on my grep foo!) due to issues in the Doxygen XML
generation when this option is enabled.

Signed-off-by: Fabio Utzig <382671175562e0df8128054dbdb05a250d79f588@nordicsemi.no>

finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr