fix(docs): add rate-limit warning to publicnode fallback RPC URL

[schoolkamsergj]

Summary

The build-app guide suggests https://base-sepolia-rpc.publicnode.com as a fallback RPC endpoint, but includes no warning about its rate limits.

The same note already advises using a dedicated RPC provider for production, but a developer could still use the publicnode endpoint in production thinking it is reliable.

Change

Added one sentence to the existing <Note> in Step 4:

This endpoint is also rate-limited and not recommended for production apps.

Before:

If `https://sepolia.base.org` is unreachable, use an alternative public endpoint
such as `https://base-sepolia-rpc.publicnode.com`. For production apps, use a
dedicated RPC provider.

After:

If `https://sepolia.base.org` is unreachable, use an alternative public endpoint
such as `https://base-sepolia-rpc.publicnode.com`. This endpoint is also
rate-limited and not recommended for production apps. For production apps, use a
dedicated RPC provider.

No other content was modified.

[cb-heimdall]

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