[docs] Add Backstage to showcase

[stefanalund]

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

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against 6fea285

[oliviertassinari]

Thanks for opening the pull request. I have cropped the image to fit.

[stefanalund]

Thanks @oliviertassinari 🙏

[oliviertassinari]

@stefanalund Thanks for putting trust in our library.

[stefanalund]

Pretty sure we are the ones who should be thankful ❤️ We have been happy users for a couple of years now for our internal version of Backstage, and more recently with the open source release. Keep up the amazing work!

[oliviertassinari]

By any change, is your organization using ag-grid? We are working on an open source and commercial alternative, we are looking for early users.

[stefanalund]

No, sorry.

[stefanalund]

@oliviertassinari Backstage still doesn't show in the list. Did something go wrong or is this expected?

[oliviertassinari]

https://next.material-ui.com/discover-more/showcase/

[oliviertassinari]

I have cherry-picked it to master, it should be live on the main domain in a couple of minutes.

[stefanalund]

Lovely -- thanks a lot 😍

[mbrookes]

@stefanalund If you could submit an image 900 X 675 that isn't cropped, that would be even better. 👍

[oliviertassinari]

I think that the dimensions should be 900 X 675 => 1200 X 900. It would match the max size we use to display the showcase when the image is rendered: 600 X 450, with a x2 grow factor to account for retina screens. I don't remember why we started with a different format for the other websites.

[stefanalund]

Fixed in #22458