docs: document python_zip_file

[dougthor42]

It looks like the python_zip_file output group is not documented. Let's document it.

filegroup(
    name = some_binary_zip",
    srcs = [":some_binary"],  # the py_binary target to make into a zip, and will be the file name
    output_group = "python_zip_file",
)

That will generate bazel-bin/path/to/target/some_binary.zip which you can then execute via python some_binary.zip. You need a system python to extract, but the zip does bundle its own interpreter (so your system python can be a different version) and is otherwise fully hermetic.