Add release notes for meshlet BVH culling (#20526)

# Objective

- To add release notes

## Solution

- Write the release notes

## Testing

- Read them

---

## Showcase

<img width="2560" height="1392" alt="meshlet-bvh-culling"
src="https://github.com/user-attachments/assets/2acc2692-a3f4-43fd-8e89-bb68c6e3a94e"
/>
(example screenshot of what should go in when we add images)

---------

Co-authored-by: atlv <[email protected]>
Co-authored-by: JMS55 <[email protected]>
Co-authored-by: Alice Cecile <[email protected]>

Author: 4 people

release-content/release-notes/meshlet-bvh-culling.md

@@ -0,0 +1,22 @@
+---
+title: Virtual Geometry BVH culling
+authors: ["@SparkyPotato", "@atlv24"]
+pull_requests: [19318]
+---
+
+(TODO: Embed example screenshot here)
+
+Bevy's virtual geometry has been greatly optimized with BVH-based culling, making the cost of rendering nearly independent of scene geometry.
+Comparing the sample scene shown above with 130k dragon instances to one with over 1 million instances, total GPU rendering time only increases by 30%.
+
+This also gets rid of the previous cluster limit that limited the world to 2^24 clusters (about 4 billion triangles).
+There are now *no* hardcoded limits to scene size. In practice you will only be limited by asset VRAM usage (since streaming is not yet implemented),
+and total instance count due the current code requiring all instances to be re-uploaded to the GPU every frame.
+
+The screenshot above has 130,000 dragons in the scene, each with about 870,000 triangles, leading to over *115 billion* total triangles in the scene.
+
+Speaking of concrete GPU cost, the scene above renders in about 3.5 ms on the 4070, with \~3.1 ms being spent on the geometry render and \~0.4 ms on the material evaluation.
+After increasing the instance count to over 1 million (almost *900 billion triangles*!), the total increases to about 4.5 ms, with \~4.1 ms on geometry render and material evaluation remaining constant at ~0.4 ms.
+This is a 30% increase in GPU time for an almost 8x increase in scene complexity.
+
+Comparing GPU times to 0.16 on a much smaller scene with 1,300 instances, previously the full render took 2.2 ms, whereas now it is 1.3 ms.