[docs] improve distributed inference cp docs.

[sayakpaul]

What does this PR do?

Our CP docs could benefit from a bit more detail and ready-to-run examples.

• Change to model checkpoint Flux.1-Dev because Qwen isn't supported for parallel yet (Context parallel bug when using QwenImage #12568).
• Add a command that should be used to invoke parallel inference.
• Simplify the example code snippet.
• Expand on how to set parallel_config.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[stevhliu]

Thanks for improving!

[stevhliu]

👍