docs: document `JJ_LOG` and `--debug` #563

Job	Run time
Build binary artifacts (linux-x86_64-musl)	6m 5s
Build binary artifacts (linux-x86_64-gnu)	5m 18s
Build binary artifacts (linux-aarch64-musl)	5m 42s
Build binary artifacts (linux-aarch64-gnu)	7m 9s
Build binary artifacts (macos-x86_64)	10m 22s
Build binary artifacts (macos-aarch64)	5m 4s
Build binary artifacts (win-x86_64)	8m 26s
	48m 6s

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