Documentation build: Avoid re-running qdoc/shiboken every time

The qdoc and shiboken steps take quite a long time when
rebuilding the documentation.

Similar to the generator dependency on
mjb_rejected_classes.log (see CMake macro
create_generator_target_library_name /
sources/cmake_helpers/helpers.cmake:262)), turn the
qdoc/shiboken targets into custom commands producing specific
files (qtcore-index.webxml, QtCore/index.rst).

This prevents the commands from being re-run as long
as the files exist and are not outdated.

Adapt the documentation.

Change-Id: I00033e6f1034a5fdc534a0f2fee018268e2e482a
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>
(cherry picked from commit 1251fb08619e030665651ea965c1ee6c283b9913)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

0 files changed, 0 insertions, 0 deletions