Organize help contents as a tree in Qt Assistant and Qt Creator

Problem
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When you open the Contents view in Qt Assistant or in the Qt Creator help mode, you get a long list of help files for Qt modules and tools. This is even worse in Qt Creator, where the documents for each installed Qt version are listed in a seemingly arbitrary order.

Solution
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Organize the help contents in alphabetic order in an expandable tree structure.

Possible Structure
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The structure should have several levels. Below is a possible structure that follows the installer structure. However, it might be useful to add "Getting Started" documentation or product-specific documentation (Qt for Device Creation, Qt for Automotive, and so on) at the top level:

Contents
Qt (Level 1)

3rd Party Documentation (L1)