[docs] Uniformize the code's font family

[oliviertassinari]

Before: https://mui.com/components/alert/#basic-alerts
After: https://deploy-preview-28582--material-ui.netlify.app/components/alert/#basic-alerts

This is an effort to simplify the font logic. We were using two different font families for the marketing and docs. We are now only using one. We are using the built-in fonts, which are more performant (to load), and closer to what developers might already be used to. However, it doesn't give us a unique brand identity feel.

We also did a couple of side cleaning:

• Remove commented code
• Remove use of font-weight: bold. Prefer theme design tokens or at the very least the 500, 600, 700, etc. scale
• Reduce the number of CSS overrides, the aim is to have a CSS -> style mapping as direct as possible

[mui-pr-bot]

No bundle size changes

Generated by 🚫 dangerJS against 440266c

[siriwatknp]

@oliviertassinari

• code font-size seems to change, is that expected? (is it not too big)
• Also, would it make sense to reduce the code font-size in mobile?

[oliviertassinari]

@siriwatknp Ok, fair enough, I have reverted the font-size change, it's 13px now on the docs. Ant Design uses 13px, GitHub 11px. VS Code, in my environment, feels like 13.5px by default React.js 14px.

[siriwatknp]

👍

[danilo-leal]

Looks good to me!