feat(docs): add developer guide for LLM APIs

This commit introduces a new guide to the documentation. 
The guide provides a comprehensive comparison of three different libraries 
for interacting with Large Language Models (LLMs): OpenAI's official API, LangChain, and LlamaIndex. 
The guide includes a detailed walkthrough of implementing a simple use case with each library, 
a comparison of the outputs, an analysis of the developer experience, 
and a discussion on the potential applications and future of LLMs. 
The guide is intended to help developers make an informed decision when choosing a library for their LLM applications.