doc: fix missing networking API documentation

Inspired by #7666, I scanned for other doxygen defgroups that weren't
mention in the documentation and found these:

   vlan  (defined in net/ethernet_vlan.h)
   ieee802154_mgmt  (defined in net/ieee802154_mgmt.h)
   ethernet_mgmt  (defined in net/ethernet_mgmt.h)

Added these to networking .rst, and also added one more level to the
toctree directive to show these nested groups in the table of contents.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>