Check link targets to Qt 5.15 docs

The doc snapshot from the master branch is now built using Qt 6.2. This breaks a lot of links to Qt reference documentation that currently use topic titles as link targets. Check all link targets and decide whether to replace them with fixed links to 5.15 docs or to remove them and the related text.

To see the QDoc warnings, select the wrench icon on the snapshot title page: https://doc-snapshots.qt.io/qtcreator-master/index.html