docs: Updating docs to source from CONTRIBUTING.md (#1850)

# What does this PR do?
Another for https://github.com/meta-llama/llama-stack/issues/1815

This links the `CONTRIBUTING.md` file directly so that we don't have to
maintain two different files.

Also I updated the title for RAG under Building AI Applications.

## Changes 
Look of what the Contributing page looks like, proof it sources directly
from the markdown file.

![Screenshot 2025-04-01 at 12 43
51 AM](https://github.com/user-attachments/assets/f7021d29-eec3-44ad-a5b3-55c4480ea9ac)

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>

docs/source/building_applications/rag.md

@@ -1,4 +1,4 @@
-## Using Retrieval Augmented Generation (RAG)
+## Retrieval Augmented Generation (RAG)
 
 RAG enables your applications to reference and recall information from previous interactions or external documents.
 

docs/source/contributing/index.md

@@ -1,14 +1,14 @@
-# Contributing to Llama Stack
 
-Start with the [Contributing Guide](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md) for some general tips. This section covers a few key topics in more detail.
+```{include} ../../../CONTRIBUTING.md
+```
+
+See the [Adding a New API Provider](new_api_provider.md) which describes how to add new API providers to the Stack.
+
 
-- [Adding a New API Provider](new_api_provider.md) describes adding new API providers to the Stack.
-- [Testing Llama Stack](testing.md) provides details about the testing framework and how to test providers and distributions.
 
 ```{toctree}
 :maxdepth: 1
 :hidden:
 
 new_api_provider
-testing
 ```