Add troubleshooting section to Nord Pool (home-assistant#36302)

Co-authored-by: Simon Lamon <[email protected]>
Co-authored-by: Franck Nijhof <[email protected]>

source/_integrations/nordpool.markdown

@@ -45,6 +45,14 @@ Data is polled from the **Nord Pool** API on an hourly basis, exactly on the hou
 
 If polling cannot happen because of no connectivity or a malfunctioning API, there is no retry; the next periodic update will try again.
 
+## Troubleshooting
+
+This service is reliant on an internet connection and that the **Nord Pool** API is available. Here are the things you can try before raising an issue:
+
+- Check that internet is available from your Home Assistant instance.
+- Check that the **Nord Pool** API is available by clicking [here](https://dataportal-api.nordpoolgroup.com/api/DayAheadPrices). You should get a JSON back with the title `Unauthorized`.
+- Use `curl` in a terminal on your Home Assistant instance using the same URL as previously opened in the browser. `curl https://dataportal-api.nordpoolgroup.com/api/DayAheadPrices`
+
 ## Sensors
 
 The integration will create entities showing today's energy prices for the configured market area. Only the base energy price is shown. VAT and other additional costs are not included.