Doc: Fix listing of Lottie examples

The .qdocconf contains a .qhp subproject definition for examples, but
it was missing from `qhp.QtLottieAnimation.subprojects`.

The two examples use an `\ingroup qtlottieexamples` command, but
the \group itself was not documented. Document it so it can be used
as an `indexTitle` for the qhp subproject.

In passing, remove nonexistent `moduleheader` from .qdocconf.
The module has only QML documentation and no C++ module header
is generated.

Pick-to: 6.12 6.11
Fixes: QTBUG-148136
Change-Id: I488a75e8a0fec69f417d06d7fe22db2b0bf6b4ee
Reviewed-by: David Boddie <david.boddie@qt.io>