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 <fabio.utzig@nordicsemi.no>