Extract examples from programmer's guide for testing

[FrederikHeber]

Most of the examples are small code snippets inside the guide and not in external files as with the user guide.

There are two approaches:

1. Extract snippets from a larger working piece through special comments in the code. Test the large piece.
2. Combine small pieces to a large piece and test.

In case 2 the programmer's guide needs to rely on files (the created snippets) in the build folder. In case 1 the test needs to combine the examples and the makefile needs to contain logic on how to combine them. This is not very maintainer-friendly.