docs: improve README.md

[melonges]

Note

Revamps README with installation steps, a modern quickstart example, authentication guidance, and a grouped method reference.

• Docs:
  • Installation: Added npm/pnpm install instructions.
  • Quickstart: New example using ClobClient with chainId, API key derivation, SignatureType, optional funder, and createAndPostOrder usage.
  • Authentication cheatsheet: Clarifies L1 wallet signature vs L2 API key, signature types, and advanced constructor options.
  • Method reference: Consolidated and grouped endpoints for:
    • Market data (public)
    • API keys and auth
    • Orders and execution
    • Trades and notifications
    • Balances and allowances
    • Rewards
    • Builder API keys
  • Points to examples/ for runnable scripts.

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