docs: fix quickstart install command

[Parikalp-Bhardwaj]

Summary

This PR updates the Rig quickstart documentation so the installation instructions match the imports used in the example.

The quickstart example imports from the public rig crate path, but the docs currently tell users to install rig-core. Installing rig-core directly would require using rig_core as the import path, so this can be confusing for new users following the quickstart.

Changes

• Updated the quickstart install command from rig-core to rig
• Added anyhow because the example uses anyhow::Error
• Added dotenvy so users can load OPENAI_API_KEY from a local .env file during development
• Added the required Tokio runtime features for #[tokio::main]
• Added dotenvy::dotenv().ok(); before creating the OpenAI client

Why

This makes the quickstart easier to copy, run, and understand for new users.

With the updated instructions, users install the same crate used by the example imports:

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

They can also create a local .env file like:
OPENAI_API_KEY=your-api-key

and the example will load it automatically with:

dotenvy::dotenv().ok();

@gold-silver-copper
Related to #88

[vercel]

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