docs(fmt): document `fmt` subcmd. #190

Job	Run time
Rustfmt (stable)	12s
Clippy (stable)	33s
cargo-deny	23s
Check (ubuntu-latest, stable)	30s
Clippy (beta)	33s
Check (ubuntu-latest, beta)	31s
Test Suite (ubuntu-latest, stable)	48s
Check (windows-latest, stable)	1m 44s
Test Suite (ubuntu-latest, beta)	55s
Check (windows-latest, beta)	1m 26s
Test Suite (windows-latest, stable)	2m 27s
Check (macOS-latest, stable)	56s
Test Suite (windows-latest, beta)	1m 47s
Check (macOS-latest, beta)	1m 0s
Test Suite (macOS-latest, stable)	1m 22s
Test Suite (macOS-latest, beta)	1m 21s
	16m 28s

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