Add example of editable package with custom src folder layout

[paulharris]

Please consider adding a more complicated example, where the user can use cmake_layout() AND add to the layout() function to support editable, where the source is in an unusual layout.

Also, this example demonstrates how to use the consumer to initialise the editable dependency build environment, which is required when the consumer may apply a complex set of options to the dependency.

I hand-tested the ci_test_example.py script, but not sure if it works on the CI. Feel free to improve, but note my goal is to improve conan documentation and examples. It isn't helpful when the documentation and examples only cover the extremely simple (cmake_layout is enough) and extremely complex (everything is customised).

[CLAassistant]

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