Doc: Clean up threading examples

- Example 3 (Clock) is a collection of anti-patterns.
    - It implements a slot in a QThread subclass and then forces the new
      thread to use Qt::DirectConnection to invoke the slot in the
      "wrong" thread.
    - It talks about getting away with non-thread-safe usage
    - It uses a thread as a timer and then admits that it's an over-
      complicated approach.
- Example 4 (Permanent Thread) is over-complicated yet incomplete. A
  better one exists in the QThread class ref.
- Example 1 (Thread Pool) is covered by the QThreadPool class ref.
- Example 2 (QtConcurrent::run()) is covered in the "Threading and
  Concurrent Programming Examples" page and the "QtConcurrentRun" page.
- The undocumented "Hello Thread" example is covered in the QThread
  class ref.
- These examples cannot be accessed from Qt Creator's Examples tool.
- These examples are neither widget-related nor tutorials, contrary to
  their source paths.

Task-number: QTBUG-33360
Change-Id: Ic79cb764ee925ddbcbeafee8e1d01db7fe0f6cfe
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Doc: Fix broken links

Task-number: QTBUG-33360

Change-Id: Ic944cb2f575c35ebad64852ef5fc44a50ac03571
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>