docs: clarify public WebSocket availability for Flashblocks

[dieutx]

Summary

• clarify that public wss://*.base.org RPC endpoints do not currently accept WebSocket upgrades
• update Flashblocks subscription docs to point readers to a node provider or self-hosted Flashblocks-aware node
• replace the direct wss://mainnet-preconf.base.org example with a provider/self-hosted placeholder

Test plan

• reviewed all impacted docs pages for consistent wording
• confirmed the branch only contains the six documentation updates for this clarification

Fixes #1409

[cb-heimdall]

🟡 Heimdall Review Status

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