rustdoc: use SVG form of Rust logo by default #92831
Description
Right now rustdoc's default logo is a PNG, which can look fuzzy on large or high DPI screens. Instead, we should use an SVG logo. For instance, there is one available at https://www.rust-lang.org/policies/media-guide. Note that it has somewhat wider margins than the PNG we currently use, so would require some tweaking to have the right size.