Consider splitting the Rust documentation (rustdoc)

The Rust part of the documentation (added as part of 9faea7f9) more than doubles the install size of linux-docs package (from ~220 MiB to ~480 MiB)

I assume this loop needs to be modified to skip the Rust stuff: https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/blob/6.13.5.arch1-1/PKGBUILD?ref_type=tags#L228-L232