Refresh RAG docs

[mrmer1]

This PR introduces a new section on Retrieval Augmented Generation (RAG) capabilities, including document grounding and citations. It provides a comprehensive guide on using RAG with Cohere's Chat API, covering various aspects such as streaming, citations, and document structure.

Changes

• A new section on RAG is added to the documentation, with three pages: Basic Usage, Streaming, and Citations.
• The page on Basic Usage provides an overview of RAG, including a quick example of calling the Chat API with RAG.
• The Streaming page explains how to enable response streaming in RAG using the chat_stream endpoint, detailing the events stream and its structure.
• The Citations page offers a guide on accessing and utilizing citations generated by the Cohere Chat endpoint for RAG, covering both non-streaming and streaming modes.
• The description of RAG is updated to emphasize its ability to minimize hallucinations when used with the Command family of models.
• The documents parameter is now described as defining the documents to be used as the context for the response.
• The id field is explained as being used in citation generation as reference document IDs, with a link to a guide on using custom IDs.
• The section on citation modes is removed from the Basic Usage page, as it is now covered in the Citations page.

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