[docs] Add notistack demo to the snackbar page

[iamhosseindhv]

Following up this issue, I have added a simple example of notistack to Snackbar documentation page.

However, this PR is not fully functional since notistack needs @material-ui/core as a peer dependency. So I wanted to make sure if I can add @material-ui/core as a devDependency to package.json ?

Thanks.

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

[joshwooding]

There is some guidance here on solving the peer dependency issue: #13659 (comment) :)

[iamhosseindhv]

Thanks @joshwooding. The issue is fixed now and live demo is working. ❤️

[oliviertassinari]

@iamhosseindhv Thank you, keep up the good work with this project :).

[iamhosseindhv]

Thanks @oliviertassinari. So great to have a demo on the official website.