add WebSocket endpoints and Flashblocks information to connecting-to-base

[Zdonuk]

Summary

Adds WebSocket endpoint information and Flashblocks block explorer reference to the "Connecting to Base" quickstart guide.

Changes

• Added WebSocket endpoints for Base Mainnet and Sepolia testnet
• Included use cases for WebSocket connections
• Added Flashblocks as an additional block explorer option with description of its features

Motivation

Developers frequently need WebSocket endpoints for real-time subscriptions and event monitoring. This information was missing from the quickstart guide. Additionally, Flashblocks provides valuable real-time analysis tools that can benefit Base developers.

[cb-heimdall]

🟡 Heimdall Review Status

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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