docs: update export private key

[RobChangCA]

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?

---

PR-Codex overview

This PR updates the documentation for exporting a user's private key using the useExportAccount hook. It emphasizes best practices and provides detailed instructions on how to implement the export functionality in an application.

Detailed summary

• Changed section title from "Usage" to "Using useExportAccount".
• Added description of the useExportAccount hook and its purpose.
• Included an import statement for useExportAccount.
• Provided example code for using useExportAccount.
• Introduced a new section titled "Using the signer".

✨ Ask PR-Codex anything about this PR by commenting with /codex {your question}

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
aa-sdk-site	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 20, 2024 4:03pm
aa-sdk-ui-demo	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 20, 2024 4:03pm