cargo: add documentation field

Allegedly this is done automatically, but I've seen so many crates
without a link to the documentation...

Cargo.toml

@@ -4,6 +4,7 @@ description = "Binding to and a wrapper for the pidfd_getfd syscall"
 license = "MIT OR Apache-2.0"
 version = "0.1.0"
 edition = "2018"
+documentation = "https://docs.rs/pidfd_getfd"
 repository = "https://github.com/PatchMixolydic/pidfd_getfd"
 keywords = ["pidfd", "linux", "syscall", "passing", "descriptors"]
 categories = ["api-bindings", "external-ffi-bindings", "os::linux-apis"]