docs: mention `testcontainers-modules` #643

Job	Run time
style	13s
test (test --all-features)	2m 50s
msrv (build --all-features)	2m 34s
test (test --no-default-features)	2m 49s
msrv (build --no-default-features)	2m 43s
lint	2m 34s
	13m 43s

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