modem: move to connectivity doxygen group

Instead of being a top-level group.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/_doxygen/groups.dox

@@ -68,6 +68,7 @@
 
 @brief Modem APIs
 @defgroup modem Modem APIs
+@ingroup connectivity
 @since 3.5
 @version 0.1.0
 @{