Add a guide to use the OpenXR controller Pointer pose

[fight4dream]

1. Create a game object named "Right Pointer Pose" under CameraRigs.UnityXRPluginFramework
2. Add Tracked Pose Driver (New Input System) component
   
3. Press the + button besides 'Position Action' and choose 'Add binding'
   
4. Select 'XR Controller' > 'XR Controller (Right Hand)' > 'Optional Controls' > 'pointerPosition'
   
5. Press the + button besides 'Rotation Action' and choose 'Add binding'
   
6. Select 'XR Controller' > 'XR Controller (Right Hand)' > 'Optional Controls' > 'pointerRotation'
   
7. Drag the 'Right Pointer Pose' to the straight pointer's 'Follow Source' field
8. Repeat the same steps for the left pointer pose.

[thestonefox]

This PR makes this easier:

ExtendRealityLtd/Tilia.Input.UnityInputSystem#39

[thestonefox]

pointerPosition and pointerRotation seem to get deleted if you use the Oculus Integration Unity asset and switch the XR Plugin to Oculus... as if the Oculus OpenXR implementation has completely removed it