[TreeView] Always apply the indentation on the item content instead of its parent's group

[flaviendelangle]

Closes #14785

• [x] Remove experimental feature
• [x] Update doc to use the new approach
• [x] Check all the customization demo to make sure it works correctly
• [x] Write migration guide

We have 3 Argos diff:

• CustomTreeItemDemo: The indentation was broken before, the new one seems a lot better
• ContentSlot and ContentSlotProps: There is no longer an indentation on the box, if we want to keep it we need to reset manually the padding on this item, but I don't think it's worth adding that complexity on this demo

[mui-bot]

Deploy preview: https://deploy-preview-15089--material-ui-x.netlify.app/

Updated pages:

• docs/data/migration/migration-tree-view-v7/migration-tree-view-v7.md
• docs/data/tree-view/rich-tree-view/ordering/ordering.md
• docs/data/tree-view/tree-item-customization/tree-item-customization.md

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