configuration_field needs a warning not to use and a link to an example of using build settings

https://docs.bazel.build/versions/3.1.0/skylark/lib/globals.html#configuration_field

We need a longer example of use. It should be put in https://github.com/bazelbuild/examples/tree/master/rules with a link in the documentation (so that we can easily test and maintain the snippet).

Start with the configuration_field page at docs.bazel.build and inspect the examples/rules directory in bazelbuild/examples. Done means adding the requested warning and documentation link, plus a longer maintainable example in the examples repository that demonstrates using build settings.

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