docs: Suggest MTU 1280 to circumvent Kabel Deutschland problems

docs/site-example/site.conf

@@ -90,7 +90,7 @@
     -- enabled = true,
     -- configurable = true,
 
-    mtu = 1426,
+    mtu = 1280,
     groups = {
       backbone = {
         -- Limit number of connected peers to reduce bandwidth.

docs/user/site.rst

@@ -138,7 +138,7 @@ fastd_mesh_vpn
         methods = {'salsa2012+umac'},
 	-- enabled = true,
 	-- configurable = true,
-        mtu = 1426,
+        mtu = 1280,
         groups = {
           backbone = {
             limit = 2,