Help fix the documentation generation toolchain

[mpusz]

There are many Modern C++ and C++20 related errors during the documentation generation process. I had to disable logs and warnings with -Q in Sphinx because Travis CI was complaining about too long build log and was terminating the build:

https://github.com/mpusz/units/blob/cb86cd2bcd8ea96f842142b1b98906fe21cf56dc/docs/CMakeLists.txt#L69

There are problems with:

• doxygen (both reported by the tool and silent problems evident in the generated XML output)
• sphinx
• breathe

We need to identify the source of the problems and report bugs for the respective tool.

[mpusz]

Some issues to follow:

• doxygen
  • https://github.com/doxygen/doxygen/issues/2732
• sphinx
  • https://github.com/sphinx-doc/sphinx/issues/7296
  • https://github.com/sphinx-doc/sphinx/issues/7296
• breathe
  • https://github.com/michaeljones/breathe/issues/479
  • https://github.com/michaeljones/breathe/issues/480
  • https://github.com/michaeljones/breathe/issues/482

[mpusz]

No longer relevant.