Add a documentation section how to open projects

In the documentation https://doc.qt.io/qtvstools/index.html there's "Managing Projects".
This could use a section that explains how to open existing qmake and CMake projects.

"Open .pro file" is briefly mentioned at https://doc.qt.io/qtvstools/qtvstools-importing-and-exporting-projects.html but that's buried somewhere at the end.

Tangent: https://doc.qt.io/qtvstools/qtvstools-importing-and-exporting-projects.html should not talk about .vcproj files anymore.