Add documentation that qt and user project build folders should be added to antivirus exception list

Building a project on Windows (both Qt and a user project) can sometimes fail due to antivirus software keeping a lock on certain files.

The failures are usually obscure error messages that don't point that it's an antivirus issue.

Multiple such reports can be found on the Qt forums.
https://www.google.com/search?q=antivirus+qt+site:forum.qt.io

The usual advice for such cases is to rerun the build multiple times until the build succeeds.

The better advice is to add the location of the build folder to the antivirus exception list.

We should have prominent documentation advising the developers to add their build folder locations to the antivirus exception list.