docs: Replace gfx-rs with wgpu in libraries section

[fryeb]

gfx-rs has been deprecated for quite some time, and wgpu is it's successor. Also wgpu is probably the most widely-used project that depends on ash (since it is used by Firefox, Bevy & countless other projects).

[MarijnS95]

There's a plan to rewrite most of the readme anyway, lots of it has gotten stale or is referencing old external links, and it's generally too verbose in demonstrating minute API details rather than describing more high-level how to build a practical Vulkan application with this (or delegate to the likes of wgpu).