docs: add example of using Circle Paymaster middleware

[ksmith-circle]

Adds an example to the third-party paymaster docs that uses Circle Paymaster to pay for gas using USDC via this npm package.

Pull Request Checklist

• Did you add new tests and confirm existing tests pass? (yarn test)
• Did you update relevant docs? (docs are found in the site folder, and guidelines for updating/adding docs can be found in the contribution guide)
• Do your commits follow the Conventional Commits standard?
• Does your PR title also follow the Conventional Commits standard?
• If you have a breaking change, is it correctly reflected in your commit message? (e.g. feat!: breaking change)
• Did you run lint (yarn lint:check) and fix any issues? (yarn lint:write)
• Did you follow the contribution guidelines?

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PR-Codex overview

This PR updates the documentation for using ERC-20 paymasters, replacing the reference to Stackup with Circle's Paymaster and providing a detailed code example for its usage with the Alchemy SDK.

Detailed summary

• Changed the paymaster reference from Stackup to Circle's Paymaster.
• Added a TypeScript code example demonstrating how to use Circle's Paymaster with the Alchemy SDK.
• Included code for generating a private key, creating a light account, and checking USDC balance.

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[rthomare]

One super small nit other wise looks good!