Add woff2 fonts and other font related tweaks (#752)

The following changes are made in this commit:

**(1)** Add the WOFF2 variant of web fonts. [WOFF2 standard](https://w3c.github.i

Add woff2 fonts and other font related tweaks (#752)

The following changes are made in this commit:

**(1)** Add the WOFF2 variant of web fonts. [WOFF2 standard](https://w3c.github.io/woff/woff2/) reduces the file sizes significantly. This is important to make our website more accessible to folks with limited Internet connections.

96% of browsers support WOFF2 ([source](https://caniuse.com/?search=woff2)) and we still have WOFF and TTF/OTF/EOT formats for those edge-cases which don't support it.

Real examples of the file size reductions for php.net:

```
Fira Sans Regular: 183 KB (woff) -> 129 KB (woff2) --> 30% reduction in size
Fira Mono Regular: 87 KB (woff) -> 65 KB (woff2) --> 35% reduction in size
```

**(2)** Use `font-display: swap` for web fonts (see [MDN Docs for font-display](https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face/font-display))

This is recommended to not block the rendering the page for a long time if the web font is not available. Again, this is important for folks with suboptimal network connections.

The `swap` value is also the default option in popular font services like [Google Fonts](https://fonts.google.com/).

**(3)** Update the list of alternative font families.

I updated the sans-serif and mono font stack used in the design. The new stacks are much longer (not a problem in practice thanks to CSS variables) but they cover all common browsing devices (including smartphones). I crafted the list from the recommended font stacks of the most popular CSS frameworks and the most popular websites for developers.

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