docs: clarify JS to Zig bridge example

[iroiro147]

Summary

• expand the Bridge docs with a concrete custom native.ping handler and dispatcher wiring
• point Web Content readers to examples/webview for the complete JavaScript-to-Zig command flow
• document the webview example flow in its README and add a changelog fragment

Closes #51.

Tests

• git diff --check
• CI=true npm exec --package=pnpm@10.23.0 -- scripts/gate.sh fast

[vercel]

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