docs need to be updated for Qt 6

http://coin.intra.qt.io/coin/doc/debugvm.html

Various things are out of date: we don't use msvc 2015 anymore, and there are no .bat files in c:\Users\qt; and basically someone needs to check step-by-step that all the steps are in place, and correct, with correct paths, on each platform.

ATM I'm trying to figure out how to build a test on Windows. I wish it would have been built already; but anyway... the docs say

To compile any test, cd into the test source directory and call ‘’‘c:\users\qt\work\qt\qtbase\bin\qmake’‘’ and ‘’‘nmake’‘’.

But in Qt 6 we use cmake. So I'll upload a screenshot of what I tried, since I can't figure out how to get clipboard text out of the VM to paste it here.