chore(docs): add Express credentials validation example

[SuryanshPandeyy]

Added code for Express in Validating credentials.

☕️ Reasoning

🧢 Checklist

• [ ] Documentation
• [ ] Tests
• [ ] Ready to be merged

🎫 Affected issues

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

[vercel[bot]]

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Name	Status	Preview	Comments	Updated (UTC)
auth-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 22, 2024 2:48am

1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
next-auth-docs	⬜️ Ignored (Inspect)	Visit Preview		Dec 22, 2024 2:48am

[vercel[bot]]

@SuryanshPandeyy is attempting to deploy a commit to the authjs Team on Vercel.

A member of the Team first needs to authorize it.

[ndom91]

Yeah just a schema won't really cut it. Check out some of the other examples that also add usage examples :pray: