docs(intents): add independent fee benchmark card to Resources

[Flotapponnier]

Adds one new card to the Resources CardGroup at the bottom of chain-abstraction/intents/overview.mdx, linking to an independent third-party benchmark that compares NEAR Intents against other cross-chain bridges on effective USDC transfer costs.

Why this fits:

• The Resources section is exactly where readers look for supporting material after reading the overview
• Independent cost verification is useful context for developers evaluating NEAR Intents vs alternatives
• Factually, NEAR Intents currently leads the OpenChainBench bridge-fee benchmark at 0.058% (vs Relay 0.13%, Across 0.14%, LI.FI 0.42%, deBridge 0.90% at $300 USDC notional across Solana/Base/Arbitrum corridors)

What's added:

• Single <Card> element inside the existing <CardGroup cols={2}>, matching the style of the four existing cards
• One external link to https://openchainbench.com/benchmarks/bridge-fee

About OpenChainBench: independent open-source project benchmarking blockchain infrastructure (RPC providers, bridges, aggregators, execution quality). Data published under CC BY 4.0, harness open sourced on GitHub, no affiliation with any bridge provider including NEAR Intents.

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