fix: Rename isFrameReady and setFrameReady

[nirwanadoteth]

What changed? Why?

Renamed deprecated isFrameReady and setFrameReady to isMiniAppReady and setMiniAppReady across all MiniKit documentation to align with the latest onchainkit API changes and deprecation notices.

Notes to reviewers

• Updated references in:
  • useMiniKit.mdx
  • overview.mdx
  • provider-and-initialization.mdx
  • add-minikit.mdx
• All code examples verified against new API naming
• Related to: coinbase/onchainkit@6d61f2b

How has it been tested?

Documentation reviewed across all files for consistency. All code examples updated and verified against the new API naming conventions.

[cb-heimdall]

🟡 Heimdall Review Status

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