docs: add simple RAG pipeline example

[shonibarej]

This PR adds a beginner-friendly example for building a simple RAG pipeline using the Conversational Retrieval QA Chain.

It includes:

• required nodes
• node connections
• explanation of embeddings and vector stores
• common beginner mistakes
• recommended prompt rules

The goal is to help new users understand and build a basic RAG chatflow more easily.

[gemini-code-assist]

Code Review

This pull request adds a comprehensive guide for building a simple RAG pipeline using the Conversational Retrieval QA Chain, covering required nodes, connections, and common pitfalls. The review feedback suggests improving the guide by incorporating a Memory node to support conversational history, clarifying the use of the Vector Store Retriever in the connection diagram, and adding the omission of memory to the list of common beginner mistakes.