docs: add pointer for AVM flash instructions

Adds a footnote with a link to an instruction on how to flash Gluon onto
AVM routers.

Signed-off-by: David Bauer <mail@david-bauer.net>

docs/index.rst

@@ -133,8 +133,8 @@ ar71xx-generic
 
 * AVM
 
-  - Fritz!Box 4020
-  - Fritz!WLAN Repeater 450E
+  - Fritz!Box 4020 [#avmflash]_
+  - Fritz!WLAN Repeater 450E [#avmflash]_
 
 * Buffalo
 
@@ -306,7 +306,7 @@ ipq40xx
 
 * AVM
 
-  - FRITZ!Box 4040 [#80211s]_
+  - FRITZ!Box 4040 [#80211s]_ [#avmflash]_
 
 * GL.iNet
 
@@ -430,6 +430,9 @@ Footnotes
   Device does not support IBSS; images are built by default unless GLUON_WLAN_MESH
   is explicitly set to something other than *11s*
 
+.. [#avmflash]
+  For instructions on how to flash AVM devices, visit https://www.fritzfla.sh
+
 License
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