docs: Add documentation on how to contribute a Vector DB provider and update testing documentation (#3093)

# What does this PR do?

- Adds documentation on how to contribute a Vector DB provider.
- Updates the testing section to be a little friendlier to navigate.
- Also added new shortcut for search so that `/` and `⌘ K` or `ctrl+K`
trigger search


<img width="1903" height="1346" alt="Screenshot 2025-08-11 at 10 10
12 AM"
src="https://github.com/user-attachments/assets/6995b3b8-a2ab-4200-be72-c5b03a784a29"
/>

<img width="1915" height="1438" alt="Screenshot 2025-08-11 at 10 10
25 AM"
src="https://github.com/user-attachments/assets/1f54d30e-5be1-4f27-b1e9-3c3537dcb8e9"
/>

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## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->

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

docs/source/contributing/testing.md

@@ -1,6 +1,8 @@
-# Testing Llama Stack
+```{include} ../../../tests/README.md
+```
 
-Tests are of three different kinds:
-- Unit tests
-- Provider focused integration tests
-- Client SDK tests
+```{include} ../../../tests/unit/README.md
+```
+
+```{include} ../../../tests/integration/README.md
+```