split "examples" into mini-examples and/or (unit) tests

[evaleev]

most examples are big and messy. Each example should become a proper tested standalone example (with matching "tutorial" in the docs walking through it), or a test (some smaller examples can be turned into proper unit tests).

use catch2

[evaleev]

@robertjharrison now with the use of FetchContent module getting Catch2 is trivial enough to be part of the official CMake docs:

https://cmake.org/cmake/help/v3.17/module/FetchContent.html#examples

target_link_libraries(unit-tests-ttg Catch2::Catch2) is all that's needed then.