Add OpenChainBench independent volume benchmark link

[Flotapponnier]

What this adds

Two small additions that surface independent, live trading data for Orderly users:

1. introduction/trade-on-orderly/trade-on-orderly.mdx — new "Track live data" section at the bottom of the overview page, with a Card linking to the OpenChainBench perp-volume-share benchmark.

2. introduction/faqs.mdx — new FAQ entry: "Where can I find independent data on Orderly's trading volume?" answering with the same benchmark link.

About OpenChainBench

OpenChainBench is an open-source, independent benchmarking site for on-chain infrastructure. The perp-volume-share benchmark tracks Orderly's 24h rolling notional volume alongside 14 other perp DEX venues, polled every 5 minutes directly from Orderly's public API (api-evm.orderly.org/v1/public/volume/stats). No proprietary data, no sign-up.

Source code: https://github.com/ChainBench/OpenChainBench

Why it fits

• Orderly users often ask where to verify volume figures independently
• The FAQ entry answers a genuine user question with a factual, verifiable source
• The Card on the overview page sits naturally alongside the existing "Learn more" resources
• Data is sourced directly from Orderly's own public API, so figures are consistent