Docs: use NUnit and DocFx to create examples

[mcavigelli]

As maintainers of the software, we want to be able to change and create the documentation with little effort.

Currently the examples use ExamplesFx, which is not maintained anymore and not widely used.
We would like to generate the documentation with NUnit as examples runner and docfx mean to write documentation. The tools should be runnable with .Net Standard 2, as we also will try to target .Net Standard with our software.

Currently the Examples are not contained in the buildconfigurations.

Tasklist

• replace ExamplesFx with an own base class
• include in build
• change structure: each example has its own directory
• extract input text with real files in each directory
• use NUnit to execute the tests
  • fix ignored tests
• save console output to console.txt file in each test directory
• include creation in build
• in examples replace file pattern to #regions in order to make it easy to target it from markdown files
• generate markdown files for each example by extracting embedded comments which targeted ExamplesFx