doc: net: ieee802154: introduce IEEE 802.15.4 subsystem doc in rst style

Moving the ieee802154 docs from a plain text file into the new rst
style. This commit only does the minimal needed change to bring the
documentation over. Follow up patches will improve and extend on this.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>