Several targets

[pthom]

First, thanks a lot for your article and for this repo. It helps a lot!

I made some simple modifications to your projet, in order to make it easier to use in the case of a standard project that would use a top "src" + "include" folder and demonstrate a use case with several targets.

This version should be compatible with the merged or not merged header project layout conventions, since it searches for headers in both folders (except src//internal src//details and src/**/private)

I do not think this version would be compatible with your article, so I do not think it should be merged into master. However - and if you are interested - may be you could add a branch with this version.

The resulting doc on readthedocs:

https://pthom-cpp-documentation-example.readthedocs.io/en/latest/

[pthom]

The advantage of this PR is that you can take cmake/FindSphinx and doc/ as they are and plug them into a new project that uses the standard "src/" and "include/" top folders, it works from the start, with no additional modification (it did on one of my project at least)