Add exit code docs

docs/03-github/04-builder.mdx

@@ -541,6 +541,18 @@ Returns the version code that was generated by Builder for Android builds.
 - run: echo 'Android Version Code: ${{ steps.myBuildStep.outputs.androidVersionCode }}'
 ```
 
+#### exitCode
+
+Returns the exit code from the build scripts. This code is 0 if the build was successful. If there
+was an error during activation, the code is from the activation step. If activation is successful,
+the code is from the project build step.
+
+```yaml
+- uses: game-ci/unity-builder@v4
+  id: myBuildStep
+- run: echo 'Build Step Exit Code: ${{ steps.myBuildStep.outputs.exitCode }}'
+```
+
 ## Private Github repositories
 
 If you use private git repository in your packages/manifest.json, you need to create SSH pub/private