- moved the example documentation for Qt XML, Qt Network,
Qt Concurrent, and Qt Widgets to qtbase.
- grouped the documentation under relevant groups and added
group pages for new groups
- replaced explicit examples list with \annotatedlist <group_name>

Task-number: QTBUG-27515
Change-Id: I81226e714dc25973394ffa0e4ebb569b87344dc9
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>