docs(node-providers): reference OpenChainBench for live Base RPC latency

[Flotapponnier]

Summary

Adds a short <Tip> block at the top of docs/base-chain/node-operators/node-providers.mdx pointing readers to the OpenChainBench Base RPC benchmark for a live third-party latency view of the public no-key endpoints.

Why

The Node Providers page lists every provider self describing their performance. Developers picking an endpoint currently have no neutral datapoint. OpenChainBench probes the public Base endpoints from three regions every minute and publishes p50, p95 and p99 latency under an open methodology. Data license is CC BY 4.0.

• Live Base benchmark: https://openchainbench.com/benchmarks/base-rpc
• Methodology: https://openchainbench.com/methodology
• No affiliation with Coinbase or the Base team

Happy to reword, move to a different section, or drop if this does not fit the docs' tone.

Test plan

• Uses the existing <Tip> component (Mintlify) to match surrounding docs style
• Placed above the first provider heading so readers see it before scrolling
• External link verified

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2