Fix: update simple RAG init to use embed_text(s) (docs)

[s3pi]

Issue

• Fixes Evaluate a simple RAG system : RAG() does not initialize any instance #2048

Changes Made

• Updated docs/getstarted/rag_eval.md to use embed_text/embed_texts.
• Added note about installing langchain-core and langchain-openai explicitly when using ChatOpenAI to avoid version mismatches.

Testing

1. make run-ci-fast

References

• Related issues: Fixes Fix: Update RAG initialization and data loading for Ragas v0.2+ (#2048) #2049
• Documentation

[anistark]

Thanks for the PR @s3pi 🙌🏼