rootless: update docs and examples #6205

Job	Run time
sandbox-build (linux/amd64)	6m 54s
build (windows/amd64)	1m 48s
sandbox-build (linux/arm64)	6m 42s
build (freebsd/amd64)	1m 48s
test-freebsd-amd64	2m 43s
test-windows-amd64 (windows-2022, containerd, ./client)	7m 53s
test-windows-amd64 (windows-2022, containerd, ./cmd/buildctl)	1m 40s
test-windows-amd64 (windows-2022, containerd, ./worker/containerd)	2m 49s
test-windows-amd64 (windows-2022, containerd, ./solver)	2m 39s
test-windows-amd64 (windows-2022, containerd, ./frontend)	4m 6s
test-windows-amd64 (windows-2022, containerd, ./frontend/dockerfile)	55m 48s
test-windows-amd64 (windows-2025, containerd, ./client)	7m 55s
test-windows-amd64 (windows-2025, containerd, ./cmd/buildctl)	2m 41s
test-windows-amd64 (windows-2025, containerd, ./worker/containerd)	2m 50s
test-windows-amd64 (windows-2025, containerd, ./solver)	2m 47s
test-windows-amd64 (windows-2025, containerd, ./frontend)	1m 39s
test-windows-amd64 (windows-2025, containerd, ./frontend/dockerfile)	49m 49s
test-windows-amd64 (windows-2022, containerd, ./..., 1)	5m 47s
test-windows-amd64 (windows-2025, containerd, ./..., 1)	4m 26s
	2h 52m 44s

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