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

Author: valeriikharkin

Cargo.toml

@@ -42,3 +42,6 @@ std = [
 	"evm/std",
 	"pallet-timestamp/std",
 ]
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]