[docs] Use slot variants over `theme.variants`

[siriwatknp]

closes #30412

Based on https://github.com/mui/material-ui/issues/30412#issuecomment-1894892080, theme.variants is no longer needed.

Using slot variants reduces the API surface, allows developers to customize any slot, and is compatible with Pigment CSS too.

• [x] I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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