feat(seo): per-chain pages v2, dimension benches + rpc-capabilities explainers

[Flotapponnier]

Summary

Follow-up to #378 (merged before this commit was pushed). Adds the v2 of per-chain landing pages:

• /benchmarks/[slug]/[chain] now supports dimension-shaped benches (chain as filter, providers as rows), not just chains-as-rows.
• 10 new pages on rpc-capabilities (one per EVM chain, hand-written explainers in the YAML).
• Regional honesty: key facts call out diverging regional leaders ("dRPC leads Ethereum RPC latency at 23 ms... Regional leaders differ: Tenderly in US-East and EU-West; dRPC in Singapore"). A provider leading a subset of chains/regions is never crowned globally.
• Chain-scoped provider leaderboard + "Leader by region" section + sibling chain nav.
• Parent bench page: "Per-chain breakdowns" nav for dimension benches.
• Sitemap: +10 rpc-capabilities chain routes.

Test plan

• Staging: /benchmarks/rpc-capabilities/ethereum renders live leaderboard + region section, self-canonical, no {{ leaks
• Parent /benchmarks/rpc-capabilities shows Per-chain breakdowns nav (hidden if bench is in draft state)
• sitemap.xml lists 20 chain routes, zero ?chain=