[material-ui][docs] Add a "start now" section on the Overview page

[danilo-leal]

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

I was revisiting some pages on the Joy UI docs and came across the nice "Start now" section we added there. Thought about doing the same here for Material UI as a slightly more inviting shortcut for doc pages than the side nav. Also pushed in some additional changes along the way:

• Moved the "Material UI vs. Base UI" section to an h3 below the "Advantages" paragraph — felt fitting.
• Removed the "...is beautiful by design" part (😅) and glued that sentence with the one above.

https://deploy-preview-41137--material-ui.netlify.app/material-ui/getting-started/

[mui-bot]

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• docs/data/material/getting-started/overview/overview.md

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