Build for only one target for docs.rs (#5427)

* Add docs.rs metadata to all cargo.toml files

* Remove docs.rs metada in substrate's cargo.toml

substrate/primitives/runtime-interface/Cargo.toml

@@ -43,3 +43,6 @@ std = [
 # Disables static assertions in `impls.rs` that checks the word size. To prevent any footgun, the
 # check is changed into a runtime check.
 disable_target_static_assertions = []
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]