C++, deduction guides

C++17 added deduction guides for class templates, https://en.cppreference.com/w/cpp/language/class_template_argument_deduction. It should be possible to document them.

Describe the solution you'd like
I'm not sure how to the should be documented, but I'm currently leaning towards a new directive cpp:deduction-guide. E.g.,

.. cpp:deduction-guide:: template<class Iter> \
                         container(Iter b, Iter e) -> container<typename std::iterator_traits<Iter>::value_type>;

The rendering should just be of the argument, without extra annotation of the object type.

Additional context

• Initiated by michaeljones/breathe#587