docs: document external neighbor list #4323

Job	Run time
Build source distribution	29s
determine-arm64-runner	1s
Build wheels for cp311-manylinux_x86_64	5m 31s
Build wheels for cp311-manylinux_x86_64	5m 7s
Build wheels for cp311-macosx_x86_64	7m 53s
Build wheels for cp311-macosx_arm64	5m 39s
Build wheels for cp311-win_amd64	6m 24s
Build wheels for cp310-manylinux_aarch64	4m 26s
build_pypi_index	14s
build_docker (12)	4m 15s
upload_pypi	0s
build_docker (_cu11, 11)	2m 42s
deploy_pypi_index	0s
Pass testing build wheels	2s
	42m 43s

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