Quick 3D: documentation for 5.14 release

The documentation for the Qt Quick 3D module needs to be in a complete state for release along with Qt 5.14 as a tech preview so that users are able to actually to use this new module and give feedback.

Getting the documentation into a releasable state should also enable us to confidently require documentation for any new features being added as a requirement for that feature being "Done".

This task consists of many sub-tasks for each part of the public API and examples (there is a second task for the examples themselves, but this task covers the documentation of all examples present currently).

All public APIs should be be fully documented. That means descriptions of what the class is used for, and each property/method, as well as helpful code snippets where appropriate as well as references to usages in example code.

In addition there should be an overview page describing the module as whole, and how to use it.

Also we need to make sure that the documentation is accessable from the global Qt Docs infrastructure (for example qtmodules.html)

For reference here are the guidelines for writing Qt documentation:
https://wiki.qt.io/Qt_Writing_Guidelines