doc: doxygen: add group definition file

Add an external group definition file. It can be used to create high
level group for things like subsystems, as there is no header such as
"subsys.h" where these type of logical groups can be defined.

This approach is followed by projects such as pipewire to organize
Doxygen modules.

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