Feedback for “Build an AI Discord Bot with Rust and Rig: Step-by-Step Guide”

This tutorial probably needs updating. I was not able to find rig::rag::RagAgent and then I see in the [CHANGELOG](https://github.com/0xPlaygrounds/rig/blob/6f12c974b2eac7d7bb1f7ac94445bd6e726d737b/rig-core/CHANGELOG.md?plain=1#L226) that it has been deprecated:

- Deprecate RagAgent and Model in favor of versatile Agent

I am not sure how to make the adjustment from RagAgent to versatile Agent.

There are also a few other items I had to change. I had to add these two imports:

use rig::vector_store::in_memory_store::InMemoryVectorIndex;
use rig::vector_store::VectorStore;

And I was not able to get the \`\`\` to compile so I have to wrap the string with r#...#.

I was not able to complete the tutorial because of the RagAgent issue, so I look forward to an update soon.

The tutorial is at 'Build an AI Discord Bot with Rust and Rig: Step-by-Step Guide'. Start by reading the CHANGELOG link to understand the deprecation of RagAgent. Look for examples of the new versatile Agent in the rig repository. Update the tutorial code to use Agent, add the necessary imports (InMemoryVectorIndex, VectorStore), and fix the string literal syntax. Test the updated steps by trying to run the bot.

索引モデルが issue の本文から書いたものです。