[material-ui][docs] Remove duplicate Design Kits page

[danilo-leal]

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

This PR removes the "Design kits" page in the "Discover more" section of the Material UI docs. I figured we didn't need it anymore as we have the "Design resources" page on the Getting Started section, which links the Kits and the Figma plugin. Also, I checked page views for this page, and it hasn't been accessed much anyway; the other one has naturally more attention as it is more prominent. So... I think we clean things up! 😃

[mui-bot]

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