docs: add WebNN execution provider reference

[Minsoo Kim (33modeling)]

Description

Adds a dedicated WebNN Execution Provider reference page for the WebNN portion of #28041. The page consolidates the provider overview, browser requirements, installation and usage, provider options, MLTensor I/O binding guidance, source-build flags, limitations, and authoritative support links.

The execution-provider index now links to the new page while the existing tutorial remains the detailed guide for browser workflows.

Validation

Documentation-only change; no runtime tests were run.

AI assistance

This documentation was drafted with OpenAI Codex assistance and reviewed before submission.

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