docs: Move the 802.11 guide into the driver-api manual

Put this documentation with the other driver docs and try to keep the top
level reasonably clean.

Cc: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/index.rst

@@ -26,7 +26,7 @@ available subsections can be seen below.
    hsi
    miscellaneous
    vme
-
+   80211/index
 
 .. only::  subproject and html
 

Documentation/index.rst

@@ -56,7 +56,6 @@ needed).
    core-api/index
    media/index
    gpu/index
-   80211/index
    security/index
    sound/index