Add trainer/head tracking input

Author: pkendall64

.wordlist.txt

@@ -24,6 +24,7 @@ Blackbox
 BluetoothLE
 Bolded
 Bootloaders
+BoxPro
 Bri'ish
 Börner
 CCPA

docs/assets/screenshots/backpack-menu.png

@@ -0,0 +1,90 @@
+---
+template: main.html
+description: ExpressLRS Bi-directional MAVLink support
+---
+
+![Software Banner](https://raw.githubusercontent.com/ExpressLRS/ExpressLRS-Hardware/master/img/software.png)
+
+## Overview
+
+The ExpressLRS TX backpack has the ability to receive channel data from external sources/backpacks e.g. the HDZero VRX backpack.
+This channel data can be sent back to EdgeTX as trainer inputs, which can then be mixed into output channels or can be configured
+to directly override channels before they are sent to a receiver.  
+
+## Requirements
+
+- ExpressLRS V4 (minimum)
+- TX Backpack 1.5.2 or later
+- EdgeTX 2.11 or later
+
+## Handset Setup
+
+In the ExpressLRS Lua script, go to the select the "> Backpack" menu entry
+
+![Backpack menu entry](../assets/screenshots/backpack-menu.png)
+
+Ensure the Backpack is turned on
+
+![Backpack enabled](../assets/screenshots/backpack-on.png)
+
+
+Scroll down to "HT Enable" and set this to "On"
+
+![HT Enable](../assets/screenshots/ht-enable.png)
+
+### Direct Channel Output
+As mentioned above ExpressLRS can override channels directly being sent to the receiver.
+To achieve this, set the "HT Start Channel" to a value from "Aux1" to "Aux10".
+
+![HT Start Aux6](../assets/screenshots/ht-aux6.png)
+
+As an example if you choose AUX6 then the channel overrides will be channels 10 upwards, depending
+on the number of channels sent via the backpack. The HDZero goggles output 3 channels for a Pan/Tilt/Roll
+head tracker so if "Aux6" is selected for the "HT Start Channel", channels 10, 11, & 12 will be overridden when
+sent to the receiver.
+
+!!! note "NOTE"
+    It is important to remember that if you must also select a packet rate and switch mode that will actually send all the
+    channels you need to the receiver. e.g if you are selecting "Aux6" then you will need to be using either 12ch or 16/2
+    channel modes
+
+### EdgeTX Trainer Input
+
+To pass the channel information from the backpack back to EdgeTX as trainer port data, set the "HT Start Channel" to
+"EdgeTX" as in the screenshot below.
+
+![HT Start EdgeTX](../assets/screenshots/ht-edgetx.png)
+
+To make the trainer channel data available to EdgeTX for using in mixes go to "Model Setup" and select "Trainer" and set
+the Mode as "Master/CRSF".
+
+![Trainer](../assets/screenshots/trainer.png)
+![Master CRSF](../assets/screenshots/master-crsf.png)
+
+Once this is done you can go to the Mixes tab and set source as a trainer input such as TR1, TR2 etc
+
+![Mixes](../assets/screenshots/mixes.png)
+
+With all this configured, channel 1 coming across the backpack should be mapped to channel 10 on the receiver.
+
+!!! note "NOTE"
+    Again, tt is important to remember that if you must also select a packet rate and switch mode that will actually send 
+    all the channels you need to the receiver. e.g if you are mapping TR1 to channel, TR2 to channel 11 etc., then you 
+    will need to be using either 12ch or 16/2 channel modes
+
+## Example Usage
+
+### HDZero Goggle head-tracking
+
+This applies to the HDZero Goggle, HDZero Goggle 2 and HDZero BoxPro+ (not the BoxPro as it does not have the backpack)
+
+- Make sure that the goggles backpack is updated with the latest version of the HDZero VRX backpack firmware
+- In the goggles menu system, go to the "ELRS" section and ensure that the Backpack is "On"
+- In the "Head Tracker" menu ensure that "Tracking" is set to "On"
+
+That should be all that is required to get head-tracking with the HDZero goggles
+
+### Boost Pedal
+
+See backpack PR https://github.com/ExpressLRS/Backpack/pull/201
+