Add `struct NoContent` as a self-described shortcut #4927

Job	Run time
cargo-hack	5m 28s
check	2m 3s
check-docs	22s
cargo-public-api-crates (axum)	31s
cargo-public-api-crates (axum-core)	23s
cargo-deny check (advisories)	27s
dependencies-are-sorted	20s
cargo-deny check (bans licenses sources)	24s
cargo-public-api-crates (axum-extra)	27s
cargo-public-api-crates (axum-macros)	18s
Spell Check with Typos	4s
test-nightly	1m 32s
test-versions (stable)	3m 5s
test-versions (beta)	3m 12s
test-msrv	1m 49s
armv5te-unknown-linux-musleabi	38s
test-docs	1m 36s
wasm32-unknown-unknown	17s
	22m 56s

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