[docs][system] Provide Custom Component demo without `styled-components`

Duplicates

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Related page

https://mui.com/system/getting-started/custom-components/

Kind of issue

Other

Issue description

Installing @mui/system tells you that Emotion is the default styling engine. It is weird to me how a few demoes later on the Custom Components page, the demo is then written with Styled Components instead.

There should be more consistency in what is recommended, and the docs should adhere to that. Demoes using other tools can (and should of course) still be included, but right now it feels like the documentation tells you "here is what we recommend, and here is why you were wrong to follow our recommendation because now we'll do it the other way".

Context 🔦

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