OmniRover: add Omni3 rover doc

Signed-off-by: Rhys Mainwaring <[email protected]>

Gazebo/docs/Omni3Rover.md

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+# Omni3 Mecanum Rover
+
+An Omni rover with 3 [Mecanum wheels](https://en.wikipedia.org/wiki/Mecanum_wheel).
+
+![omni3rover](https://github.com/user-attachments/assets/53d5d0bf-e4e3-4f03-a50a-4004a9865309)
+
+
+## Usage
+
+#### Run Gazebo
+
+Edit the `omnirover_playpen.sdf` to include the `omni3rover` model.
+
+```bash
+gz sim -v4 -r omnirover_playpen.sdf
+```
+
+#### Run ArduPilot SITL
+
+```bash
+sim_vehicle.py -v Rover --model JSON --add-param-file=$HOME/SITL_Models/Gazebo/config/omni3rover.param --console --map
+```
+
+Once armed, set the mode to `MANUAL` and control as follows:
+
+| Channel | Low | High |
+| --- | --- | --- |
+`rc 1` | yaw left | yaw right |
+`rc 2` | not assigned | |
+`rc 3` | move back | move forward |
+`rc 4` | move left | move right |
+
+
+## Links
+
+- [Rover wiki: Motor and Servo Connections: Omni-Vehicles](https://ardupilot.org/rover/docs/rover-motor-and-servo-connections.html#omni-vehicles)