Document the status and outlook on support of modern C++ in Qt

The purpose of this new section or page in the Qt documentation is to provide an overview to users on how Qt adheres to C++ features and standards.

At the moment, this information is avaialble only in form of blogs and postings on mailing lists. Still, in many cases, users may realize out of sudden that a given Qt version cannot be compiled with an available toolchain. This new page should give users orientation about the major aspects and timings of the C++ support.

Despite of an outlook on the known short-term plans, it currently make sense to give an overview of major facts which C++ standard were supported in Qt5 as well