Update ERR_NGROK_8012 documentation and add details file

docs/errors/details/_err_ngrok_8012.md

@@ -0,0 +1,31 @@
+### Overview
+
+This error occurs when the ngrok agent successfully receives traffic but fails to establish a connection with your local service (the "upstream" service). While the tunnel is created successfully, ngrok cannot forward the traffic to your application.
+
+### Common Issues and Solutions
+
+1. **Local Service Not Running or Inaccessible**
+
+   **Cause:** The web service or application on your local machine is not active or cannot be reached.
+
+   **Resolution:**
+
+   - Start your local service.
+   - Confirm you can access it directly (e.g., http://localhost:8080).
+   - Restart ngrok with the correct port number.
+
+2. **Incorrect Port Specified**
+
+   **Cause:** The port in your ngrok command doesn't match your local service's port.
+
+   **Resolution:**
+
+   - Verify the port your local service is using.
+   - Update your ngrok command to use the correct port.
+   - Example: Use `ngrok http 3000` if your service runs on port 3000.
+
+### General Troubleshooting Steps
+
+1. Verify local service accessibility without ngrok.
+2. Ensure the latest ngrok version is installed.
+3. Restart the ngrok agent.

docs/errors/err_ngrok_8012.mdx

@@ -15,4 +15,9 @@ Traffic was successfully tunneled to the ngrok agent, but the agent failed to es
 
 ### Further help
 
-If you're having trouble resolving this error, please reach out to [[email protected]](mailto:[email protected]?subject=Help%20with%20ERR_NGROK_8012)
+If issues persist after trying these solutions, please contact [[email protected]](mailto:[email protected]?subject=Help%20with%20ERR_NGROK_8012) with:
+
+- Exact ngrok command used
+- Operating system and version
+- Full error message or screenshot
+- Steps to reproduce the problem