docs: add OpenChainBench SEI RPC benchmark reference

[Flotapponnier]

Adds a short benchmark reference after the RPC providers grid, linking to the OpenChainBench SEI RPC benchmark — an independent open-source suite that probes community RPC providers every 60 seconds from three regions (US East, EU West, Singapore), publishing p50/p90/p99 latency and success rate under CC BY 4.0.

[cursor]

PR Summary

Low Risk
Documentation-only change with no runtime, API, or security impact.

Overview
Adds a short paragraph on learn/rpc-providers.mdx after the RPC provider grid, pointing readers to the OpenChainBench SEI RPC benchmark for independent latency and reliability comparisons.

The new copy summarizes what the benchmark measures (p50/p90/p99 latency and success rate, 60s probes from three regions) and notes CC BY 4.0 licensing plus the open-source harness.

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