Add README for coin-gated web example

[raymondjacobson]

Summary

• Adds a README to packages/web/examples/coin-gated/ covering setup, env vars, project layout, and SDK usage
• Adds coin-gated entry to the parent packages/web/examples/README.md table

Test plan

• Verify README renders correctly on GitHub

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 243fd54

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[github-actions]

🌐 Web preview ready

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