Doc: Move threading overviews from qtbase.git to qtdoc.git

- This topic is  relevant to multiple modules, as illustrated by the
  "Thread-Support in Qt Modules" page. Multithreading can be done in
  both C++ and QML.
- Moving also fixes links to QML-related pages.
- All files are transferred unchanged, except for "\snippet" commands.
  The folder hierarchy is slightly different here.

Change-Id: I1717b243a4837bcdd95f3bb22f5ad7adcb3f793e
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>