Document GitLab CI integration #2859

Job	Run time
test-llvm (14)	1m 24s
test (stable)	1m 49s
test-llvm (15)	1m 28s
test (beta)	1m 52s
test-llvm (16)	1m 39s
test (nightly)	1m 17s
test-llvm (17)	1m 39s
test (nightly, macos-latest)	2m 50s
test-llvm (18)	57s
test (nightly, windows-latest)	4m 28s
test-llvm (19)	1m 5s
deny / deny	14s
miri / miri	2m 16s
msrv / msrv	3m 4s
release-dry-run / prepare	5s
tidy / tidy	1m 6s
release-dry-run / build (aarch64-unknown-linux-gnu)	1m 30s
release-dry-run / build (aarch64-unknown-linux-musl)	1m 14s
release-dry-run / build (aarch64-apple-darwin, macos-13)	1m 52s
release-dry-run / build (x86_64-unknown-linux-gnu)	1m 5s
release-dry-run / build (x86_64-unknown-linux-musl)	1m 6s
release-dry-run / build (x86_64-apple-darwin, macos-13)	2m 29s
release-dry-run / build (x86_64-pc-windows-msvc, windows-2019)	3m 0s
release-dry-run / build (x86_64-unknown-freebsd)	1m 43s
release-dry-run / build (universal-apple-darwin, macos-13)	3m 4s
	44m 16s

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