[add] Troubleshooting section for API calls

[ll-zerr]

Cover some troubleshooting for API calls that are not returning the data as expected.
Things like:

• Problem: No data is displayed when sending a GET request
  Possible Solution: Verify that data exists in the collection for the call to return a result. If there are no entries in the collection, you need to add an entry to populate that collection with data. Then you should be able to see data with the GET request.

• Problem: Only first-level data is being returned on a GET request, and you need second-level data or relations data.
  Possible Solution: Pass the populate=* parameter (or combinations of it) to populate deeper levels of data within the collection info returned. See this Strapi docs link for more details.