doc(r3): add width and height to the root element of logo-large-bg.svg

Fixes it being rendered too small on <https://lib.rs/crates/r3>.

It appears that lib.rs rasterizes SVG images (instead of preserving
vector data, which GitHub and crates.io do) and looks at its `viewBox`
to determine the output size if `width` or `height` is missing.

Author: yvt