[plan docs] WebGPU EP extraction plans (#32213)
### Description
<!-- Describe your changes. -->
Add plans for extracting the WebGPU EP code into its own repo.
Starting point is the overview doc:
[`docs/design/webgpu_ep_extraction/webgpu_ep_extraction.md`](https://github.com/microsoft/onnxruntime/blob/edgchen1/webgpu_ep_extraction_plan/docs/design/webgpu_ep_extraction/webgpu_ep_extraction.md).
There are several distinct workstreams with their own docs too.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Decouple WebGPU EP development from the ORT repo.
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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Copilot-Session: df9d994f-d4ed-4e61-8104-57abb42ae205
