doc: fix missing networking API documentation
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 <c56c8b29908a5b2af1b63001268d73a0b7f972ba@intel.com>

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