Documentation Request: Clarify required setup steps for OnchainKitProvider in README

### Is your documentation request related to a problem? Please describe.

When following the current README to set up `OnchainKitProvider`, it is not always clear which configuration fields are required versus optional. As a result, developers may omit critical values (such as an API key or network configuration) and encounter confusing runtime errors that do not clearly point back to missing setup steps.

Start by reviewing the README section that explains OnchainKitProvider setup and identify each configuration field described there. Update the documentation to distinguish required from optional values, including the API key and network configuration, then verify that the setup instructions clearly explain what must be provided.

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