docs: reorganize contributor guide (#3110)

**Description:** 
Restructures contribution guide and move some sections into categories

<img width="1399" height="527" alt="Screenshot 2025-08-12 at 9 28 44 AM"
src="https://github.com/user-attachments/assets/404e23b4-0001-4174-b662-593e0173ef7d"
/>

docs/source/contributing/index.md

@@ -2,17 +2,6 @@
 ```{include} ../../../CONTRIBUTING.md
 ```
 
-## Testing
-
-See the [Test Page](testing.md) which describes how to test your changes.
-```{toctree}
-:maxdepth: 1
-:hidden:
-:caption: Testing
-
-testing
-```
-
 ## Adding a New Provider
 
 See the [Adding a New API Provider Page](new_api_provider.md) which describes how to add new API providers to the Stack.
@@ -27,3 +16,14 @@ See the [External Provider Page](../providers/external/index.md) which describes
 new_api_provider
 new_vector_database
 ```
+
+## Testing
+
+See the [Test Page](testing.md) which describes how to test your changes.
+```{toctree}
+:maxdepth: 1
+:hidden:
+:caption: Testing
+
+testing
+```