docs: add OnchainKitProvider setup guide to README

[devacc8]

Summary

Added a comprehensive setup section for OnchainKitProvider to both README files to help developers quickly understand required vs optional configuration.

Changes

• Added "OnchainKitProvider Setup" section after Quickstart
• Basic usage example with required props
• Configuration options table with Required/Optional markers
• Config object structure documentation
• Note about API key requirements for Paymaster and Identity features

Why

Developers were having trouble distinguishing required from optional fields, leading to cryptic runtime errors when critical setup steps were skipped.

Configuration Table Preview

Prop	Required	Description
chain	✅ Yes	Target chain
children	✅ Yes	Your app components
apiKey	Recommended	CDP API Key
projectId	Optional	CDP Project ID
rpcUrl	Optional	Custom RPC URL
config	Optional	Appearance and wallet config
miniKit	Optional	Mini App configuration

Fixes #2563

[vercel]

@devacc8 is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[cb-heimdall]

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