docs: add missing documentation for SNPE, VSINPU, WebGPU, and WebNN execution providers

### Description

The following execution providers have code implementations in the ONNX Runtime repository but lack documentation pages on the [onnxruntime.ai](https://onnxruntime.ai/docs/execution-providers/) documentation site:

| Execution Provider | Code Location | Current Docs |
|---|---|---|
| **SNPE** | `onnxruntime/core/providers/snpe/` | None |
| **VSINPU** | `onnxruntime/core/providers/vsinpu/` | None |
| **WebGPU** | `onnxruntime/core/providers/webgpu/` | Tutorial only (`docs/tutorials/web/ep-webgpu.md`) — no dedicated EP page |
| **WebNN** | `onnxruntime/core/providers/webnn/` | Tutorial only (`docs/tutorials/web/ep-webnn.md`) — no dedicated EP page |

### Request

Add proper EP documentation pages (on the `gh-pages` branch under `docs/execution-providers/`) for each of these providers, following the existing EP page format which typically includes:
- Overview and supported platforms
- Build instructions / prerequisites
- Configuration options and provider-specific settings
- Supported operators / known limitations
- Usage examples

### Context

This was discovered during a documentation audit comparing all EP code directories against the gh-pages documentation. WebGPU and WebNN are particularly high-impact since they are actively used web platform EPs with only tutorial-level docs.

For SNPE and VSINPU, input from the respective EP maintainers would be needed to write accurate documentation.

Start by reviewing the existing pages under docs/execution-providers/ on the gh-pages branch, then compare them with the SNPE, VSINPU, WebGPU, and WebNN code directories and the existing WebGPU/WebNN tutorials. Done means four provider pages covering the requested sections, with maintainer input for SNPE and VSINPU where needed.

Written by the indexing model from the issue text.