Documentation issue: Common C Build Use Cases

Documentation URL: https://docs.bazel.build/versions/4.1.0/cpp-use-cases.html

The sections `Including external libraries` and `Writing and running C++ tests` are untestable in the current state.

1. The `./test/hello-test.cc` file includes `#include "lib/hello-greet.h"`, but nowhere was hello-greet mentioned. I guessed it was referring to the structure in [stage3 of the previous tutorial](https://docs.bazel.build/versions/4.1.0/tutorial/cpp.html#refine-your-bazel-build), but `hello-greet.h` in that tutorial is in `main/` and not `/lib`.

I ended up creating a directory structure like


which managed to work, but it would be nice to add this to the [github cpp-tutorial](https://github.com/bazelbuild/examples/tree/master/cpp-tutorial), maybe as a 4th stage.

2. The other problem is that the first WORKSPACE + gtest.BUILD (without the strip_prefix) in https://docs.bazel.build/versions/4.1.0/cpp-use-cases.html#including-external-libraries didn't compile for me. I get the error:


Start with the Common C++ Build Use Cases page, especially the “Including external libraries” and “Writing and running C++ tests” sections, and compare them with the github cpp-tutorial stages. Reproduce the hello-greet layout and the WORKSPACE/gtest.BUILD example, then update the documentation or tutorial so both examples compile and their required file structure is explicit.

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