Otherwise on systems where Arial is not available the system will
fallback to a serif font, rather than a sans-serif one.

This is especially relevant on acessibility-conscious setups (such as is
mine) that have web-fonts disabled and a limited set of fonts available
on the system.

On most platforms and browsers, `sans-serif` is equivalent to Arial.
However, on Firefox on Ubuntu (and possibly other Linuxes), `sans-serif`
is DejaVu Sans, a much wider font. This creates a larger shift in text
when the custom fonts finally load. Arial is a web-safe font, and
specifying it explicitly gives us more cross-platform consistency, as
well as reducing the layout shift that happens when fonts load.

Add font-display: swap. Per https://web.dev/font-display/, this prevents
"flash of invisible text" during load by using a system font until the
custom font is available. I've noticed this flash of invisible text
occasionally when reading Rust docs.

Add an explicit height to icons (which already had an explicit width)
to allow browsers to lay out the page more accurately before the icons
have been loaded. https://web.dev/optimize-cls/.

Add min-width: 115px to the crate search dropdown. When the HTML first
loads, this dropdown includes only the text "All crates." Later, JS
loads the items underneath it, some of which are wider. That causes
the dropdown to get wider, causing a distracting reflow. This sets a
min-width based on the size that the dropdown eventually becomes based
on the crates on doc.rust-lang.org, reducing page movement during load.

For browsers that support woff2 (most modern ones:
https://caniuse.com/woff2), this offers a reduction in download size
for these two fonts from 362k to 257k (32% reduction). It decreases the
total page size for `struct.String.html` (counting all subresources) by
about 2.5%.

If this is interesting, I'm happy to apply the same treatment to the
other fonts, but these two are the biggest.

Fix #81377

This fixes a mobile UI bug where a vertical scrollbar would always be
rendered on the sidebar nav when the menu was closed.

Fixes an issue where links in the one-line version of an item's docs
would be in Fira Sans, while the rest would be in a serifed font.

I had put it in the wrong file in #76126. This should fix it now. Thank
you to @ollie27 for pointing this out!

* Rename it in the UI
* Rename the CSS classes