ECOSYSTEM.md: add link to rust-api.dev #4771

Job	Run time
cargo-hack	3m 7s
check-docs	20s
cargo-public-api-crates (axum)	30s
check	47s
cargo-public-api-crates (axum-core)	23s
cargo-public-api-crates (axum-extra)	32s
cargo-deny check (advisories)	24s
dependencies-are-sorted	16s
cargo-public-api-crates (axum-macros)	20s
Spell Check with Typos	7s
cargo-deny check (bans licenses sources)	25s
test-msrv	3m 29s
test-versions (stable)	1m 18s
test-versions (beta)	1m 27s
test-docs	1m 29s
armv5te-unknown-linux-musleabi	24s
test-nightly	1m 7s
wasm32-unknown-unknown	32s
	16m 57s

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