Reload the page after changing the font size

Summary:
Currently the font size chage does not work because the content is not updated.

BUG: 415192

Test Plan:
1. Open some help file.
2. Click on the "Increase/Decrease font size" buttons.
3. The size of the font should change.

Reviewers: #kde_applications

Subscribers: kde-doc-english

Tags: #documentation

Differential Revision: https://phabricator.kde.org/D27046