Combine the six examples associated with each tutorial chapter
into a single, top-level example project 'extending-qml', with
subprojects for each of the tutorial chapters.
Clean up the docs, add links, and a note about a warning that
the user may see when running the code in the first chapter.
Task-number: QTBUG-32947
Change-Id: Idba4e2153817ab29f1afaf1947d1f2e25964e7b3
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
fccf0e2912