net: wifi: Enhance wifi_mgmt documentation

Enhance wifi_mgmt group documentation by :
 - Hiding some internal entities i.e. the human-readable strings
   defined in the various static const char * const arrays, by
   marking them as @cond INTERNAL_HIDDEN blocks.
 - Moving the Wi-Fi management API page under the networking group and
   adding a human readable name.

Signed-off-by: Sachin D Kulkarni <sachin.kulkarni@nordicsemi.no>