Rollup merge of #82545 - jsha:woff2, r=GuillaumeGomez

Rollup merge of #82545 - jsha:woff2, r=GuillaumeGomez

rustdoc: add optional woff2 versions of FiraSans.

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.