Add "Made by KDE" logo

Summary:
This change adds a "Made by KDE" logo into the site header. The intention of this is to improve the brand visibility of KDE.
See T9122 for more details on the effort.

Test Plan: - Works fine with Mobile & Desktop

Reviewers: Falkon, drosca

Reviewed By: Falkon, drosca

Subscribers: Falkon

Differential Revision: https://phabricator.kde.org/D15104