docs: reference weather radar map for europe for outdoor_chanlist

The map is generated from EUMETNET data, it's source code can be
found at https://gitlab.com/aaaaalbert/wxr-channel-map.

docs/user/site.rst

@@ -188,6 +188,12 @@ wifi5 \: optional
     When set to ``true`` all 5 GHz radios will use outdoor channels, while on ``false``
     the outdoor mode will be completely disabled. The default setting is ``'preset'``,
     which will enable outdoor mode automatically on outdoor-capable devices.
+
+    It can be beneficial to look up the WLAN channels that are used by `weather radars`_
+    when constructing ``outdoor_chanlist`` to try and minimize the impact of DFS events.
+
+    .. _weather radars: https://homepage.univie.ac.at/albert.rafetseder/RADARs/help.html
+
     ::
 
       wifi5 = {