docs: add docs for using undici dispatcher

[blowsie]

🔗 Linked issue

#379, #215, #316

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

agent setting only applies to older versions of node.
As of version 18 node fetch is native and depends on undici, which uses a dispatcher option instead

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[blowsie]

https://undici.nodejs.org/#/docs/api/ProxyAgent?id=example-basic-proxy-request-with-local-agent-dispatcher

[pi0]

Thanks!