Dark + Light SVG versions of rust-social-wide.jpg

### Summary
Add an SVG version of [/static/images/rust-social-wide.jpg](../tree/master/static/images/rust-social-wide.jpg) with dark and light variants.

### Motivation
For use in https://github.com/rust-lang/rust/pull/123093

### Rationale and alternatives
Alternatively, these SVGs could be added directly to the `rust-lang/rust` repo where they would be used.