Use bold italic at 100% for sect4 titles, and bold 100% for sect5 titles

Summary:
Now the titles are small and hard to distinguish.

BUG: 419256

Test Plan:
Replace kde-docs.css in your system to see the results.

Before:
{F8199642}
After:
{F8199643}

Reviewers: #documentation, danders

Reviewed By: danders

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

Tags: #frameworks, #documentation

Differential Revision: https://phabricator.kde.org/D28304