Use the new shared library exported by KDocTools

Summary:
The only users are kio_help/kio_ghelp, so there should be no
API/ABI concerns.

Test Plan:
It compiles against the shared KF5::DocTools library, and kio_help
still works.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D4850