Unblock font loading in rustdoc.css

Unblock font loading in rustdoc.css

rustdoc's font loading defaults to "auto", so browsers may block render.
But rustdoc's case prefers a faster TTI for scrolling, this means the
strictest font-display in use should be "swap". rustdoc's fonts do provide
notable legibility improvements but first-time users will have little trouble
reading without. This means "optional" is preferred.

The one exception is Source Serif Pro: it's a big difference for body text, so
"fallback" is preferred over "optional" to cause a (tiny) block.