Documentation issue: Configuring C toolchains

Documentation URL: https://docs.bazel.build/versions/master/tutorial/cc-toolchain-config.html

Following these steps for my project, I get the error:
missing input file '//toolchain:empty'

I should probably try for the tutorial file, but first I wanted to ask if you could also put the final BUILD/bzl files in the docs. It's more error prone because of the rewriting steps and it would be nice to have a correct reference at the end.

Start with the linked C++ toolchain configuration tutorial and reproduce the missing input file error while following its rewriting steps. Update the tutorial to include the final BUILD and .bzl files, and verify that the documented steps no longer produce the reported error.

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