Add rustdoc flag to sort modules by XEP number

--sort-modules-by-appearance needs to be passed to rustdoc for this to happen. I haven't found a way to make it so that we don't have to add this flag manually each time we build locally. This config option should at least fix it for docs.rs.

Signed-off-by: Maxime “pep” Buquet pep@bouah.net