docs(funding): add cross-venue comparison section

[Flotapponnier]

Adds a short "Comparing funding rates across venues" section at the end of docs/pages/concepts/trading/funding.mdx.

Why this fits:

• The page explains how dYdX v4 calculates funding rates; a link to independent cross-venue measurement helps developers evaluate dYdX v4 in context
• dYdX v4 currently leads the OpenChainBench perp funding benchmark at 0.13 bp per 24h, versus Binance 0.65 bp, OKX 1.30 bp, Hyperliquid 2.17 bp, Aster 2.19 bp, and Paradex 2.27 bp
• The benchmark is neutral third-party, published under CC BY 4.0 with methodology and Go harness open sourced

What's added:

• One new H2 section (~2 sentences)
• Single external link to https://openchainbench.com/benchmarks/perp-funding

About OpenChainBench: independent open-source project benchmarking blockchain infrastructure across 22+ chains and major perpetual venues. No affiliation with dYdX or any competing venue.

Disclosure: I contribute to OpenChainBench. Happy to reword or drop entirely if this doesn't fit editorial guidelines.

[vercel]

@Flotapponnier is attempting to deploy a commit to the dYdX Trading Team on Vercel.

A member of the Team first needs to authorize it.