[docs] Revise Showcase copy for clarity + audit appList

[samuelsycamore]

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

This PR follows up on the conversation from #31942, bringing more clarity to the Showcase page.

https://deploy-preview-31946--material-ui.netlify.app/discover-more/showcase/

Additionally, the copy shifts the branding away from MUI as a product to MUI as a company with a suite of products. While the text is not technically true today—we only have one component library as I'm writing this—I think this is acceptable given the imminent release of Base, plus it makes the copy more future-proof so it won't need to be updated again anytime soon.

[mui-bot]

Details of bundle changes

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Generated by 🚫 dangerJS against 6edd3df

[danilo-leal]

I guess that in order to have this new copy live, we should probably check whether the projects we currently have match our requirements. I'm sure that there are a bunch there that don't.

[samuelsycamore]

That is a good point @danilo-leal. Do you think that auditing the current listings should be a part of this PR, or should that be a separate issue?

[danilo-leal]

Making the auditing part of this PR will probably be safer in terms of making sure the content is following what we required... But doing it separately would also work, just a matter of coordinating them I guess.

[samuelsycamore]

I reviewed all sites listed and removed those that were broken, outdated, or apparently no longer using Material UI.

What do we think about super old apps on the list using < v5? I found a couple v1 examples, not sure whether they're worth keeping for the historical record. 😛

[samuelsycamore]

I completely forgot about this PR—could've sworn it was already merged! 😅

[danilo-leal]

Looking great!