wireless: move documentation books

This moves mac80211 documentation into a new
802.11 bookset and also adds a cfg80211 book
to the set. All of this is rather incomplete,
but it's easier to work with big code moving
as a separate patch.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>