Update design to look more similar to kde.org

Summary:
This patch mostly change the CSS to look more similar to the one from
kde.org/kontact.kde.org/...

The files common/en/{top-left.jpg,top-right.jpg,kde_logo_bg.png}
are now deprecated and will be removed in KF6.

Screenshots:

Test Plan: Tested with not regenerated documentation and regenerated documentation

Reviewers: VDG, Documentation, yurchor, ltoscano

Reviewed By: Documentation, yurchor, ltoscano

Subscribers: yurchor, rooty, ltoscano, bruns, abetts, broulik, aacid, kde-frameworks-devel, kde-doc-english

Tags: Frameworks, Documentation

Differential Revision: https://phabricator.kde.org/D19557