Add preloading hints for default fonts

[workingjubilee]

This adds preloading hints to rustdoc's HTML for certain fonts
that are always loaded from the server. This enables browsers
to improve loading performance of the page.

This is an attempt to improve on the situation discussed in #20962 and also for historical interest it is following up on lessons learned in #72557 (comment)

Browsers lazy-load fonts in @font-face rules. The generation, calculation, and styling of a std doc page takes entire seconds, whereas loading a font takes milliseconds. It's hard to know when exactly the fonts are requested, but examining results from webpagetest.org suggests it could take entire seconds into the page load. Using <link> for all of rustdoc's webfonts might be unwise (it would be wasted loads if the fonts are local), and CSS extensions can override these, but as long as the CSS is unaltered we can be confident a visitor to a rustdoc page will want these fonts.

[steveklabnik]

r? @rust-lang/rustdoc

[workingjubilee]

Reviewing the theme and extension logic here, I am more uncertain about this PR being correct now, and could use guidance on how to assure the assumptions it rests upon are in fact correct.

[Dylan-DPC-zz]

Had a discussion with the reviewer, and given the uncertainty around this, i'm closing this pr.