- added toc
- fixed typo in outlook section
- fixed typo in subparser section
- fixed typo in sub-parser section
- fixed formatting of option-documentation
- fixed description of sub-parser example
- Clarified introduction of sub-parsers.
- clarify default value
- clarification of subparser section
- added two newlines in description of purpose

- Clearified build instructions (dependencies)
- Fixed gcovr instructions
- Clearified sanizier section.
- Updated installation instructions to be generator-independent.
- Removed coverage section, as it is already part of build instructions.

- type of basic_option_t::_value changed from std::optional<T> to T.
- `operator bool()` now returns `_value != default_value` instead of `has_value()`.

Added new architectures and OSs:
- includes amd64 (debian 10, ubuntu 20.04, ubuntu 21.04)
- includes arm64 (debian 10, ubuntu 20.04, ubuntu 21.04)
- includes armhf (debian 10)

furhtermore, several other improvements were done:
- added new compiler warnings for gcc-10 and gcc-11 for pedantic builds
- created new build-targets (i.e. libClaPP_CXX_COMPILE_OPTIONS) which contain
  all warnings for the current compiler.

This was required, as clang-7 formatted code leads to
`compound-token-split-by-space` warning in clang-12.