rootless: update docs and examples #5556

Job	Run time
prepare	1s
test / prepare	2m 12s
test / run (containerd, ./frontend/dockerfile, integration)	5m 40s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 34s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 47s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	7m 16s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 51s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 36s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	5m 13s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	7m 25s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 53s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 40s
test / run (oci, ./frontend/dockerfile, integration)	4m 46s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 34s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 26s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 48s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	5m 15s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	6m 2s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 14s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 20s
image (mainline)	5m 28s
image (labs)	5m 49s
release	0s
scout	0s
	2h 4m 50s

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