doc: add qa doc including test case for GET/categories/all

docs/quality_assurance_test_cases.md

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+This document contains some examples of test cases for each feature implemented on the backend. 
+
+**Tools:**
+
+- Test the REST API with Swagger UI provided in the link of the deployed server.
+
+- Test the REST API using Postman (if you feel comfortable with this).
+
+**Notes:**
+
+- Outcome *Fail* means the test case has no effect in the database, so no changes are done in the data. An error message should be returned. 
+
+- Outcome *Success* means that the test case was successful and had an effect in the database, so this change/effect should be reflected in the database.
+
+# Access to unrestricted APIs
+
+- This part deals with the possible test cases while accessing Unrestricted APIs.
+
+## Categories 
+
+- The only unrestricted APIs are: GET/categories/all,  GET/categories/{id}/sections
+
+### Get all sections
+
+**Service:** GET/categories/all
+
+**Parameters:** None
+
+| Test Case | Outcome |
+| --------- | --------- |
+| Fetch sections for all existing category ids | Success |