Rename directory docs/preparetips to preparetips5

rename directory + adapt docs/CMakeLists.txt

Rationale:

Translated docbooks in several frameworks are missing in the tarballs, see https://todo.kde.org/?controller=task&action=show&task_id=520

To find out which subdir of docs/frameworks subdir it should grab translated
docs from, the frameworks release script uses the name of the subdir in the sources.

On the other hand the script autogen.sh in l10n-kf5/scripts generates the CMakeLists.txt files
with the correct install paths/names for the language docbooks using the dirnames in l10n/[lang]/documentation.

Now the folder in the sources has a different name than the install name in the CMakeLists.txt,
in this case dirname preparetips != install name preparetips5.

Therefore doc/preparetips in kconfigwidgets is pulled into l10n-kf5/scripts/documentation
with the name preparetips5 using an entry in documentation_paths in l10n-kf5/scripts
and then processed by scripty and used by translation teams.

As discussed with David renaming the dirs in the frameworks repos is the easiest solution and does not require
ugly hacks either in frameworks release script or in autogen.sh.
REVIEW:127904