squashed many small commit messages for documentation and matrix/ci i…

[KjellKod]

…mprovements

PULL REQUEST DESCRIPTION

ADD CONTENT HERE TO DESCRIBE THE PURPOSE OF THE PULL REQUEST

Formatting

• I am following the formatting style of the existing codebase.

a clang-format configuration file is available in the root of g3log.

• Use VSCode with clang-formatter or commandline:
  clang-format -i path_to_file
• or recursive throughout the whole repo: find . -iname "*.hpp" -o -iname "*.cpp" | xargs clang-format -i

Testing

• This new/modified code was covered by unit tests.
• (insight) Was all tests written using TDD (Test Driven Development) style?
• The CI (Windows, Linux, OSX) are working without issues.
• Was new functionality documented?
• The testing steps 1 - 2 below were followed

step 1

mkdir build; cd build; cmake -DADD_G3LOG_UNIT_TEST=ON ..

// linux/osx alternative, simply run: ./scripts/buildAndRunTests.sh

step 2: use one of these alternatives to run tests:

• Cross-Platform: ctest
• or ctest -V for verbose output
• Linux: make test

[KjellKod]

reviewed, and we will NOT merge this but force push after squash to master