Optional usage of documentationDirs from KDocTools

Summary:
Only if available. Unfortunately a bug in KDocTools prevents for
looking for its version, so use another the version of another core
library part of Frameworks.

Test Plan:
The new code compiles without regression in functionalities:
- the cache is created for toc entries
- the documentation is found.

Reviewers: aacid, cfeck

Reviewed By: aacid

Subscribers: #documentation

Tags: #documentation

Differential Revision: https://phabricator.kde.org/D6790