[docs][toolpad] Remove extra code block

[bharatkashyap]

From #43796 (comment)

Preview: https://deploy-preview-44120--material-ui.netlify.app/material-ui/react-dialog/

[mui-bot]

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• docs/data/material/components/dialogs/dialogs.md

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[oliviertassinari]

The demo is quite hard to follow, I thought the point was that this code could be simpler and shown before the demo.

[prakhargupta1]

and shown before the demo.

or, as a preview? Like we show in most demos?

[oliviertassinari]

or, as a preview? Like we show in most demos?

@prakhargupta1 I don't understand this, right now in https://deploy-preview-44120--material-ui.netlify.app/material-ui/react-dialog/#toolpad-beta, we have noise:

and when we open the demo, we have to struggle to find the piece that I want to copy and paste in my project.

---

I think that the right move is to:

1. Keep the simplification to the demo done in this PR.
2. Not show the inline preview, there is nothing to show AFAIK (like in HEAD).
3. Update the code example of https://mui.com/material-ui/react-dialog/#toolpad-beta to match with 1.
4. Move the code example to be before the demo as the demo is only for first-time learning, once you learned the feature, and you repeatedly come to the docs, you need this bloc to copy and paste.
5. So also add the missing pieces to the code example to make it self-sustained (e.g. imports), but while keeping as simple as possible, simpler than the demo

[siriwatknp]

@bharatkashyap Do you mind changing the <Button loading> back to <LoadingButton>? it will be added in v7 instead. https://mui-org.slack.com/archives/C02BDJNTKF1/p1732593650637169