Update Cloud LLM brick docs

[91volt]

Summary

This PR enhances the documentation for the Cloud LLM Brick to provide better guidance for users and clearer API definitions.

What changed

• Markdown Documentation:
  • Rewrote the Overview and Features sections for better clarity.
  • Added detailed Configuration and Supported Models tables.
  • Added instructions on how to setup LLM api key.
  • Improved code examples for basic chat and streaming with memory.
  • Added a summary of available Methods.
• Python Code:
  • Updated class and method docstrings to include precise argument types, return values, and raised exceptions.
  • Added docstrings for internal helpers like model_factory.

[mcmchris]

LGTM!