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/bin/node/rpc-client/Cargo.toml

@@ -15,3 +15,6 @@ jsonrpc-core-client = { version = "14.0.3", features = ["http", "ws"] }
 log = "0.4.8"
 node-primitives = { version = "2.0.0-alpha.5", path = "../primitives" }
 sc-rpc = { version = "2.0.0-alpha.5", path = "../../../client/rpc" }
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]