Source all C++ code examples in the documentation from files in the repo and build them as examples

[RippeR37]

The idea here is to source all the example C++ code snippets from example-files inside the code repository and ensure that all of them are building correctly and are +/- self-sufficient.

There are ways to manipulate what is included from the file, so parts of the examples could be omitted and the in-documentation examples could link to the full standalone mini-programs with that example code for standalone full examples.

That would also prevent problems where there is a typo, or wrong syntax mistakenly added to some examples.