Added DT IPTV setup

Setup guide for Deutsche Telekom Germany Magenta TV IPTV

source/manual/how-tos/dt_ger_iptv.rst

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+**Deutsche Telekom Germany IPTV (Magenta TV) setup**
+============================
+
+**Original Author:** Jascha Kirchhoff
+
+**Introduction**
+-----------------
+This guide is for setting up Deutsche Telekom Germany IPTV (Magenta TV) and assumes you already have a working internet connection and the
+os-igmp-proxy plugin installed.
+
+This is just a basic working setup. You can separate all IPTV traffic into a VLAN, if needed.
+
+All network hardware between OPNsense and the Media Receiver or TV Box must support IGMP snooping. Without IGMP snooping enabled, the network 
+gets flooded with multicast traffic and live tv starts stuttering.  
+
+
+**IGMPproxy setup**
+-------------------
+
+Ensure you are running OPNsense 22.1 or later
+
+Then configure IGMPproxy as follows
+
+.. image:: images/dt_ger_iptv_01.png
+    :width: 100%
+
+.. image:: images/dt_ger_iptv_02.png
+    :width: 100%
+
+
+NOTE: downstream interface is LAN 
+
+.. image:: images/dt_ger_iptv_03.png
+    :width: 100%
+
+
+**FIREWALL setup**
+------------------
+
+We need to add two new rules for the WAN interface and modify one (the default IPv4 rule) on the LAN to get Magenta TV working. The key is
+to enable "allow options" in the Advanced Options for all three (!) rules, WAN and LAN.
+
+.. image:: images/dt_ger_iptv_04.png
+    :width: 100%
+
+NOTE the Source is "*"
+
+.. image:: images/dt_ger_iptv_05.png
+    :width: 100%
+	
+.. image:: images/dt_ger_iptv_06.png
+    :width: 100%
+
+.. image:: images/dt_ger_iptv_05.png
+    :width: 100%
+
+And finally the outbound NAT
+
+.. image:: images/dt_ger_iptv_07.png
+    :width: 100%
+	
+I switched to Hybrid mode but it should also work in Automatic mode, because none of the outbound rules need to be modified.
+
+Make sure you have clicked Save & Apply
+
+It is advisable at this point to reboot the system.
+
+Plug in your Media Receiver to one LAN port, turn on the receiver and after a few minutes you should see live TV. Also software updates should
+work out of the box. Update mode has been tested 2022-05, no additional settings are required.