Update WebNN architecture diagram SVG (#871)

Mermaid Live Editor export produces an invalid SVG.
To fix this, we apply the following patch:

perl -pi -e 's/ / /g' webnn_arch.png

This replaces all non-breaking space   character entity
references with numeric entity references  

Mermaid syntax does not support numeric entity references,
so we need to rely on this patch if we want to be white
space perfect.

Consider:
Core ML => Core ML

Mermaid source used to produce this SVG:

```mermaid
%%{ init : { "look" : "handDrawn", "theme" : "neo-dark" }}%%
flowchart TD
  subgraph Models
    Model[ONNX, TensorFlow, PyTorch]
  end

  subgraph Frameworks
    Framework(ONNX Runtime Web, LiteRT)
  end

  subgraph Browser_APIs[Browser APIs]
    Wasm
    WebNN
    WebGPU
  end

  subgraph Native_ML_APIs[Native ML APIs]
    MLAPI[DirectML / LiteRT / Core ML]
  end

  subgraph Hardware
    CPU
    NPU
    GPU
  end

  Model --> Framework

  Framework --> Wasm
  Framework ==> WebNN
  Framework --> WebGPU

  Wasm --> CPU
  WebNN ==> MLAPI
  WebGPU --> GPU

  MLAPI ==> CPU
  MLAPI ==> NPU
  MLAPI ==> GPU
  ```

Author: anssiko