[Docs] Add some WiFi related information to docs

Last week I added this text in a discussion on an issue.
It is also useful in the documentation so keep a copy there.

docs/source/WiFi/WiFi.rst

@@ -0,0 +1,25 @@
+WiFi
+****
+
+WiFi disconnect reasons
+=======================
+
+Beacon timeout (200)
+--------------------
+
+For more information on the Beacon frame, see `Wikipedia <https://en.wikipedia.org/wiki/Beacon_frame>`_.
+
+In short, the access point sends periodically a packet containing information about the network.
+This interval is typically 100 TU (102.4 msec).
+The ESP module is trying to listen to this beacon every time, but for a number of reasons it may
+miss a beacon frame.
+The timeout is longer than 100 TU, so it must miss a number of these beacon frames
+to report a beacon timeout.
+Reasons to miss such a beacon frame are:
+
+- too busy processing other blocking tasks (very likely)
+- access point not sending a beacon due to high traffic demands of others (depending on brand/model/settings)
+- RF disturbances (not likely given how often this occurs)
+- clock drift (not really likely)
+
+So the "beacon timeout" is happening every now and then on the ESP nodes and ESPeasy will try to reconnect.

docs/source/index.rst

@@ -15,6 +15,7 @@ Welcome to ESPEasy's documentation!
    Controller/Controller.rst
    Plugin/Plugin.rst
    Rules/Rules.rst
+   WiFi/WiFi.rst
 
 .. raw:: latex