Clarify public WebSocket availability in Flashblocks docs

## Summary
Several Base docs pages currently imply that the public `wss://*.base.org` endpoints can be used directly for Flashblocks subscriptions.

## What I observed
A WebSocket upgrade request to the public Base RPC endpoints returns `405 Method Not Allowed` instead of establishing a subscription connection.

## Impacted areas
- RPC overview
- `eth_subscribe`
- Flashblocks FAQ
- `newFlashblocks`
- `newFlashblockTransactions`
- `pendingLogs`

## Suggested fix
Update the docs to clarify that:
- the public `base.org` RPC endpoints are currently HTTP-only
- subscription-based Flashblocks usage should go through a node provider or a self-hosted Flashblocks-aware node
- examples should avoid suggesting direct use of `wss://mainnet-preconf.base.org`