Doc: Ensure all pages are listed in the help project

Topics for `lottietoqml` tool and the `qt_target_qml_from_lottie` CMake
command were not listed in the .qhp table of contents generated by QDoc.

Add a qhp subproject of type `manual`, and a \list structure that
links to them.

In addition, the \qmlmodule page for Qt.labs.lottieqt.VectorImageHelpers
was orphaned; not explicitly linked from anywhere. Modify the `Qt Lottie
Animation QML Types` page to list also types from the submodule, and
add a dedicated .qhp subproject for them.

This ensures that all pages generated by QDoc are also reachable from
Contents in Qt Creator/Assistant, as well as the topic tree in the
online documentation.

Pick-to: 6.12
Change-Id: I8b621f820a68a5126bca3f79d3826ca8251d59b5
Reviewed-by: Alexei Cazacov <alexei.cazacov@qt.io>