[core] Batch small changes

[oliviertassinari]

• [core] Fix typo 139f724
• [core] prop-types import convention 3a00eee: a follow-up on Fix prop-types imports #26566 (comment)
• [docs] Consistent terminology 5c8ebf3: settle on "Material Design guidelines" over the alternatives. cc @mbrookes
• [docs] Show the correct import path ae339bd:
• [docs] Fix /customization/density/ demo 318ec00: the initial value was "8px" instead of 8.
• [docs] Remove orphan files 3c60cde:
• [docs] No GA in the packages 0195200: the confusion Difference between Lab and Core components? #19448 (comment)
• [docs] Avoid confusion of version supported 639a725: the confusion https://twitter.com/rachanapand/status/1396506371919544320

[mui-pr-bot]

No bundle size changes (experimental)

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[oliviertassinari]

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