MacOS info for all books

I propose to discuss this and decide on a solution for all the guides and
all the writers, via the list and/or the documentation team weekly meeting.

*If* macOS customisation is critical then please give more consideration to
coding it in the documents. It won't end there, scripting guides could be
customised for preferred programming language.

If we add a control variable that hides or show sections/paragraphs
depending on the OS, documents will go into a very complex and fragile
management scheme where mistakes (e.g. deletion of a controlled
paragraph) are easily introduced and unnoticed. Debugging a long
document like our guides will be unpleasant at best.

Technically speaking, LibreOffice has all tools for document management
(version control, fields, track-changes...) but our team must get used
to and it is also a entry barrier for newcomers.

The Help has a control scheme (<switch>, <switchinline>) for macOS where
Ctrl key is changed to Command key and the menu "Tools-Option" is
changed to "LibreOffice - Preferences". Those are 99.99% of all
differences in Help between OS versions.