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/frame/staking/Cargo.toml

@@ -71,3 +71,6 @@ runtime-benchmarks = [
 	"rand_chacha",
 	"frame-benchmarking",
 ]
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]

substrate/frame/staking/fuzz/Cargo.toml

@@ -33,3 +33,6 @@ members = ["."]
 [[bin]]
 name = "submit_solution"
 path = "fuzz_targets/submit_solution.rs"
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]

substrate/frame/staking/reward-curve/Cargo.toml

@@ -19,3 +19,6 @@ proc-macro-crate = "0.1.4"
 
 [dev-dependencies]
 sp-runtime = { version = "2.0.0-alpha.5", path = "../../../primitives/runtime" }
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]