[docs] Refine Toolpad sections on Material UI docs

[Janpot]

Summary

• Don't frame it as "Experimental API" [docs] Link Toolpad Core components from Material UI docs material-ui#43036 (comment)
  Proposal: Name all these sections "Toolpad"
• Add example for notifications and dialogs [docs] Link Toolpad Core components from Material UI docs material-ui#43036 (comment)
  Proposal: Make it similar to the notistack example. Indicate well that this is not native Material UI

• Don't call it a "framework" here: https://mui.com/material-ui/getting-started/templates/#toolpad-core-beta

[oliviertassinari]

Proposal: Name all these sections "Toolpad"

For Dialog, Snackbar, it can make sense to me. Long-term these would be in Material UI? It's the layout component that felt strange.

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