More detailed simulation download instructions

This explicitly notes the relationship between the user-code and
the simulation files.

Fixes #397
Fixes srobo/competition-simulator#280

Author: PeterJCLaw

simulator/index.md

@@ -28,8 +28,19 @@ You will also need Python installed.
 
 ### Installing the simulation
 
-1. [Download the simulation](https://github.com/srobo/competition-simulator/releases/download/sr2023.1/competition-simulator-sr2023.1.zip), and unzip it somewhere on your computer.
-2. Using the Webots IDE, open the `worlds/Arena.wbt` file.
+1. Create a directory, perhaps called `simulation` where you will store your robot code.
+2. [Download the simulation](https://github.com/srobo/competition-simulator/releases/download/sr2023.1/competition-simulator-sr2023.1.zip)
+  and unzip it as a subdirectory of that directory:
+  ```
+  simulation
+  ├── competition-simulator-<version>
+  │   ├── ...
+  │   └─ worlds
+  │       └── Arena.wbt
+  └── robot.py
+  ```
+  If there is not an existing `robot.py` an example one will be created when the simulator first runs.
+3. Using the Webots IDE, open the `worlds/Arena.wbt` file.
 
 You may receive a warning about your computer's GPU not being good enough, which can be ignored.