docs: add SwiftNodes to third-party RPC providers list

[swiftnodes]

Summary

Adds SwiftNodes to the third-party RPC providers reference table for Arbitrum.

SwiftNodes operates a flat-rate multi-chain RPC service that includes Arbitrum One (chain ID 42161) over both HTTP and WebSocket. The row follows the existing table format and links to our Arbitrum endpoint page.

Column	Value	Notes
Arb One?	✅	eth_chainId → 0xa4b1 (42161)
Arb Nova?	—	not currently served
Arb Sepolia?	—	not currently served
Websocket?	✅	wss:// endpoint available
Stylus Tracing?	—	not advertised

Only truthful capabilities are checked. Happy to adjust wording or placement per maintainer preference.

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