mdhelp: add network help docs

docs/class/mpegts_network.md

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+A network is the type of carrier for your television signals. Tvheadend
+supports several different types of network, notably:
+
+* Cable TV, delivered via a cable to your house
+  - [DVB-C](class/dvb_network_dvbc)
+  - [ISDB-C](class/dvb_network_isdb_c)
+  - [ATSC-C](class/dvb_network_atsc_c) - common in north and central America and parts of south Asia
+
+* Satellite, any signal coming in via a dish
+  - [DVB-S](class/dvb_network_dvbs)
+  - [ISDB-S](class/dvb_network_isdb_s)
+
+* Terrestrial, over-the-air broadcasts received through a traditional television aerial
+  - [DVB-T](class/dvb_network_dvbt) - common in most of Europe
+  - [ISDB-T](class/dvb_network_isdb_t) - common in Brazil and various other countries throughout south America
+  - [ATSC-T](class/dvb_network_atsc_t) - common in north and central America
+
+* IPTV - TV over the Internet via your broadband connection
+  - [IPTV](class/iptv_network)
+  - [IPTV Automatic Network](class/iptv_auto_network) - IPTV using a playlist as the source
+
+!['Networks' Tab Screenshot](docresources/dvbinputsnetwork.png)
+
+---
+
+###Add/Edit Dialog Example
+
+A common set of fields is used for the _Add_ or _Edit_ functions, most
+of which can also be seen in the grid view:
+
+!['Add/Edit Network' Dialog - DVB-S/2](docresources/dvbnetworkedit.png)
+
+!['Add/Edit Network' Dialog - IPTV](docresources/configdvbnetwork_iptv.png)
+
+---
+
+###Menu Bar/Buttons
+
+The following functions are available:
+
+Button         | Function
+---------------|---------
+**Save**       | Save any changes made to the network configuration.
+**Undo**       | Undo any changes made to the network configuration since the last save.
+**Add**        | Add a new network. You can choose from any of the types described above.
+**Delete**     | Delete an existing network. This will also remove any association with an adapter.
+**Edit**       | Edit an existing network. This allows you to change any of the parameters you’d otherwise set when adding a new network, e.g. network discovery, idle scan, etc. - similar to using the check boxes to enable/disable functions.
+**Force Scan** | Force a new scan (i.e. scan all muxes for services) for the selected networks.
+**Help**       | Displays this help page. 
+
+---