[docs-infra] Add stray design adjustments

[danilo-leal]

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

General run-through:

• Fine-tune the branding theme's solid button styles, removing the gradient and using box shadows.
• Polishing up the gradient Demo background, removing many fuzzy radial gradients. Ended up going for one-off colors as I wouldn't be able to pull off a smooth gradient with opacities from existing colors. It's not 100% ideal, but it seems worth it for this specific use case!
• Refined the chips from the component link header, reducing the icon size a bit, as well as fine-tuning spacing.

Examples:

• https://deploy-preview-40623--material-ui.netlify.app/
• https://deploy-preview-40623--material-ui.netlify.app/base-ui/react-button/

[mui-bot]

Netlify deploy preview

• docs/data/material/guides/material-3-components/material-3-components.md

Bundle size report

No bundle size changes (Toolpad)
No bundle size changes

Generated by 🚫 dangerJS against e2aa92b

[zanivan]

Left some comments, but those are ultimately a matter of opinion, so feel free to merge it when you think it's ready 🚀

[oliviertassinari]

Oh, on ButtonBase, this is ambitious. There will likely be a lot more reset needed, #40747 sounds like only the tip of the iceberg.

[danilo-leal]

I've circled around both the marketing pages and the documentation, and aside from the instance in your PR, nothing crazy stood out. In any case, do let me know if you find any further regressions! 👍 Many buttons/links were missing proper focus state treatment prior to this.

[oliviertassinari]

Many buttons/links were missing proper focus state treatment prior to this.

@danilo-leal True, this might make for the best tradeoff. We never thought about this use case for ButtonBase when introducing it, nice 😄.

find any further regressions

I mostly saw broken cases around tabs.