[docs] Migrate Portal demos to emotion

[vicasas]

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

Related to #16947

[mui-pr-bot]

No bundle size changes

Generated by 🚫 dangerJS against d886032

[oliviertassinari]

I'm leaving a note for @mnajdova. This made me wonder if we should document

import Portal from '@material-ui/unstyled/Portal';

instead of

import Portal from '@material-ui/core/Portal';

or if we should add an ## Unstyled hero section in the page.

[mnajdova]

I'm leaving a note for @mnajdova. This made me wonder if we should document

import Portal from '@material-ui/unstyled/Portal';

instead of

import Portal from '@material-ui/core/Portal';

or if we should add an ## Unstyled hero section in the page.

For the sake of simplicity from developer's perspective, I think we should leave the import

import Portal from '@material-ui/core/Portal';

Developers shouldn't care about the specifics of whether this component has styles or not. If they want to use the unstyled version, they may change the import to the unstyled package. We anyway aim to have an unstyled version for all components. +1 for adding some kind of Unstyled tag or something similar in the docs that indicates that this is an unstyled component.

[oliviertassinari]

In this case, I would propose we add an ## Unstyled section on these pages with a mention to

import Portal from '@material-ui/unstyled/Portal';

While the unstyled nature of the demos on the page suggests, it's not obvious that it's available in this package too.