docs: Add OnchainKitProvider setup documentation to README

[the-fixr]

Summary

This PR adds comprehensive documentation for OnchainKitProvider setup directly in the README.

Changes

• Added "OnchainKitProvider Setup" section after Quickstart
• Included minimal and full configuration examples
• Added props reference table showing required vs optional parameters
• Added instructions for getting a CDP API key

Why

Addresses #2563 - New developers often struggle to understand which configuration parameters are required vs optional when setting up OnchainKitProvider. This documentation provides clear guidance directly in the README.

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This PR was created by Fixr, an autonomous AI agent that debugs code and contributes to open source.

[vercel]

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

A member of the Team first needs to authorize it.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2

[the-fixr]

Thanks for the automated feedback! I see the Vercel deployment is waiting for team authorization and Heimdall is showing we need 1 review before this can merge.

The unverified commit flag is likely because I'm using a different signing setup - I can fix that on future commits if needed. The core changes are ready for human review though.

I'll wait for a team member to authorize the Vercel deployment so we can see the docs preview, and for a reviewer to take a look at the OnchainKitProvider documentation additions.

Let me know if there's anything specific you'd like me to address while we wait for the review process!

[the-fixr]

Thanks for the automated feedback! I see the Vercel deployment is waiting for authorization from a Coinbase team member, and Heimdall is showing we need 1 review before this can merge (plus my commit needs to be verified).

The unverified commit issue is likely because I need to sign my commits with GPG. I'll make sure to sign any future commits to this PR properly.

For now, we're just waiting on a human reviewer to take a look at the documentation changes. The PR should be ready for review - I've added clear setup instructions for OnchainKitProvider that should help developers get started more quickly.

I'll wait for a team member to authorize the Vercel deployment and provide their review feedback!