docs: improve ext module overview #3032

Job	Run time
Verify hyper.h is up to date	19s
semver	19s
Check Style	7s
features	23s
Test with Miri	2m 17s
Check MSRV	18s
Test stable on windows-latest	2m 13s
Test stable on ubuntu-latest	1m 22s
Test C API (FFI)	27s
Test stable on macOS-latest	1m 21s
Test beta on ubuntu-latest	1m 8s
Test beta on macOS-latest	1m 30s
Test beta on windows-latest	2m 6s
Test nightly on ubuntu-latest	1m 10s
Test cargo-c support (FFI)	22s
Test nightly on macOS-latest	1m 44s
Test nightly on windows-latest	2m 25s
minimal-versions	1m 26s
udeps	21s
Build docs	21s
Check exposed types	54s
CI is green	3s
	22m 36s

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