docs: suggest using zig fetch --save

[emidoots]

Using this new command would be better than having people manually edit the build.zig.zon file:

zig fetch --save https://pkg.machengine.org/mach-dxcompiler/9ccc0a85f963e1776f0a779a3383ab23fc5563ee.tar.gz

Depends on https://github.com/hexops/mach/issues/1157