[Khelpcenter] Use HiDPI icons

Summary: Use HiDPI icons when scaling is used

Test Plan:
Before:
{F5882879}

After:
{F5882877}

Reviewers: davidedmundson, lueck

Reviewed By: davidedmundson

Subscribers: kde-doc-english

Tags: #documentation

Differential Revision: https://phabricator.kde.org/D13265