Docs: Enhance contributor guide with testing section (#18852)

Updated the contributor guide to include testing information.

## Which issue does this PR close?

- part of #7013

## Rationale for this change

Similarly to #18851, 

I was trying to find the right documentation to point people (and AI
tools) at for running the tests wanted to make it easier to find the
testing instructions immediately on the contributing landing page:
https://datafusion.apache.org/contributor-guide/index.html#development-environment

<img width="672" height="153" alt="Screenshot 2025-11-20 at 12 45 35 PM"
src="https://github.com/user-attachments/assets/403d7047-fe37-4c78-b108-f56870a5c9ab"
/>


## What changes are included in this PR?

Add a link to the testing page directly 

## Are these changes tested?

I ran it locally

## Are there any user-facing changes?

<!--
If there are user-facing changes then we may require documentation to be
updated before approving the PR.
-->

<!--
If there are any breaking changes to public APIs, please add the `api
change` label.
-->

Author: alamb

docs/source/contributor-guide/index.md

@@ -32,7 +32,8 @@ community as well as get more familiar with Rust and the relevant codebases.
 
 ## Development Environment
 
-You can find how to setup build and testing environment [here](https://datafusion.apache.org/contributor-guide/development_environment.html)
+Setup your development environment [here](development_environment.md), and learn
+how to test the code [here](testing.md).
 
 ## Finding and Creating Issues to Work On