KF5DocToolsMacros.cmake: Use non-deprecated KDEInstallDirs variables

Summary:
HTML_INSTALL_DIR -> KDE_INSTALL_DOCBUNDLEDIR
MAN_INSTALL_DIR -> KDE_INSTALL_MANDIR

BUG: 410998

Test Plan: Install paths are correct by example of marble.

Reviewers: Frameworks, ltoscano

Reviewed By: ltoscano

Subscribers: ltoscano, kde-frameworks-devel, kde-doc-english

Tags: Frameworks, Documentation

Differential Revision: https://phabricator.kde.org/D23229