[docs] Exclude demos from next.js eslint rules

[Janpot]

The lint rules shouldn't apply to the demos

See mui/mui-x#10619

[mui-bot]

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[Janpot]

This change was proposed to fix issues in MUI X where a demo contains <a> tags with a path-relative url to a docs page. These are problematic because

1. they won't work in codesandbox
2. they initiate a full page reload

ignoring this error is not the right way forward, instead:

1. if it's not intended as a demo and more as a internal component: hide the demo toolbar and instead use next.js <Link /> components. This will avoid the full page reload
2. if it's intended as a demo, then we can simply supply absolute urls to the <a> tags. This will have the same behavior as today, and also will work inside of codesandbox