This cuts the HTML overhead for a page by about 1KiB,
significantly reducing the overall size of the docs bundle.

According to <https://caniuse.com/?search=svg%20favicon>,
SVG favicons are supported in everything but Safari.

When I actually try it in Safari, it's downloading all
three favicons, and nothing looks different when I disable
the 16x16 one.

<https://dev.to/masakudamatsu/favicon-nightmare-how-to-maintain-sanity-3al7>,
which is linked from caniuse above, recommends an ico.
However, the reason they recommend it is the apps that
only support /favicon.ico exactly, and rustdoc can't assume
it will be installed to the site root, so it's unfortunately
up to the webmaster to make sure it's set up.