Add usage section to `zarrs_reencode` docs #162

Job	Run time
fmt	27s
clippy	35s
build (stable, --features "")	1m 17s
test	1m 31s
build (stable, --all-features)	2m 58s
build (stable, --no-default-features)	1m 19s
build (1.80, --features "")	1m 35s
build (1.80, --all-features)	3m 13s
build (1.80, --no-default-features)	1m 32s
build (beta, --features "")	2m 15s
build (beta, --all-features)	3m 41s
build (beta, --no-default-features)	2m 42s
build (nightly, --features "")	2m 12s
build (nightly, --all-features)	3m 15s
build (nightly, --no-default-features)	3m 3s
	31m 35s

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