Add release note

Author: hukasu

release-content/release-notes/release_notes_template.md

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+---
+title: Render Assets diagnostics
+authors: ["@hukasu"]
+pull_requests: [19311]
+---
+
+## Goals
+
+Create diagnostics plugins `MeshAllocatorDiagnosticPlugin`, `MaterialDiagnosticPlugin` and `RenderAssetDiagnosticPlugin`
+that collect measurements related to `MeshAllocator`s, `MaterialBindGroupAllocator`, and `RenderAssets` respectively.
+
+`MeshAllocatorDiagnosticPlugin` and `MaterialDiagnosticPlugin` measure the number of slabs, the total size of memory
+allocated by the slabs, and the number of objects allocated in the slabs. Only bindless materials use slabs for their
+allocations, non-bindless materials return 0 for all of them.
+
+`RenderAssetDiagnosticsPlugin` measure the number of assets in `RenderAssets<T>`.