[DOC-FIX] LLM RAG Evaluation notebooks are missing information

[lz-chen]

Willingness to contribute

Yes. I would be willing to contribute a document fix with guidance from the MLflow community.

URL(s) with the issue

https://mlflow.org/docs/latest/llms/llm-evaluate/notebooks/rag-evaluation.html

Description of proposal (what needs changing)

• This documentation is missing important information on how to set up the llm for evaluation. It would be useful to link the AI gateway documentation here.
• In the last cell
  results.tables["eval_results_table"] does not work. results.tables is empty for some reason

[github-actions]

@mlflow/mlflow-team Please assign a maintainer and start triaging this issue.