docs.rig.rs examples out of sync with rig-core 0.36.0 (from_env Result + cli_chatbot usage)

Hi! I just started experimenting with Rig and ran into a couple docs examples that do not compile for me.

I am using `rig-core = 0.36.0`.

## 1) Quickstart example: `Client::from_env()` result is not handled

Page: `https://docs.rig.rs/docs/quickstart/getting_started`

Current snippet:

```rust
use rig::client::{CompletionClient, ProviderClient};
use rig::completion::Prompt;
use rig::providers::openai;

#[tokio::main]
async fn main() -> Result<(), anyhow::Error> {
// Create OpenAI client
let client = openai::Client::from_env();

// Create agent with a single context prompt
let comedian_agent = client
.agent("gpt-5.2")
.preamble("You are a comedian here to entertain the user using humour and jokes.")
.build();

// Prompt the agent and print the response
let response = comedian_agent.prompt("Entertain me!").await?;

println!("{response}");

Ok(())
}
```

`Client::from_env()` returns `Result`, so this would need a `?` to propagate the error up or explicit error handling.

## 2) CLI chatbot example uses old API shape

Page: `https://docs.rig.rs/docs/extensions/cli_chatbot`

Current snippet:

```rust
use rig::{cli_chatbot, providers::openai};

let agent = openai.agent("gpt-4")
.preamble("You are a helpful assistant.")
.build();

cli_chatbot(agent).await?;
```

To my knowledge, `cli_chatbot` is a module, not a callable function in `rig-core 0.36.0`.
Suggested fix is builder-based:

```rust
use rig::integrations::cli_chatbot::ChatBotBuilder;

// ...
let chatbot = ChatBotBuilder::new().agent(agent).build();
chatbot.run().await?;
```

If this looks right, I’m happy to open a PR with these fixes. Should I go ahead?

The issue points to two specific documentation pages on docs.rig.rs. The first is the quickstart guide where `Client::from_env()` needs error handling. The second is the CLI chatbot extension page where the API usage is outdated. Check the current rig-core 0.36.0 source to confirm the correct API, then update the MDX files in the documentation repository accordingly. Verify the changes by building the docs locally.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.