Refresh backend README with progress timeline (#20115)

[JulianCloudNTH]

Summary:

Update the WebGPU backend README to reflect the current state of the
backend:

• Add a Progress section listing milestones landed on main
  ([ETVK] WebGPU runtime #18808, [ExecuTorch][WebGPU] Add rms_norm op #19963, [ExecuTorch][WebGPU] Enable backend test suite + x86 CI #19964, Generate *_wgsl.h embedded shaders from *.wgsl (#19981) #19981, [ExecuTorch][WebGPU] Fix CI: link shared wgpu-native to avoid LTO bitcode clash #20036) and work in review
  ([ExecuTorch][WebGPU] Switch native backend from wgpu-native to Dawn (Tint) + SwiftShader #20079, [ExecuTorch][WebGPU] Add per-pass dispatch ordering + scratch buffer tests #20080), each linking its pull request.
• Update the operator support table to include rms_norm and refresh
  the planned/roadmap list toward end-to-end LLM inference.
• Update the directory structure to match the current layout.

Docs-only change; no code or build impact.

Differential Revision: D107742574

[meta-codesync]

@JulianCloudNTH has exported this pull request. If you are a Meta employee, you can view the originating Diff in D107742574.

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