[docs] Update contribution guide to mention when to use REP (#30844)

This is a small change. Our current contribution guide doesn't mention how to propose features / larger changes. I spotted the issue when processing https://discuss.ray.io/t/using-ray-over-infiniband/5883/9

Signed-off-by: Zhe Zhang <zhz@anyscale.com>

doc/source/ray-contribute/getting-involved.rst

@@ -28,6 +28,7 @@ We welcome (and encourage!) all forms of contributions to Ray, including and not
 - Code readability and code comments to improve readability.
 - Test cases to make the codebase more robust.
 - Tutorials, blog posts, talks that promote the project.
+- Features and major changes via Ray Enhancement Proposals (REP): https://github.com/ray-project/enhancements
 
 What can I work on?
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