Update fr.kde.org to use the Aether theme

Summary:
Use the Jekyll KDE theme

TODO:

• remove made by kde https://invent.kde.org/websites/jekyll-kde-theme/commit/126f7528b483125de6b7e9a223ce03a75373905a
• fix navbar in the jekyll theme

Test Plan:

Reviewers: ervin

Reviewed By: ervin

Subscribers: jjazeix, ervin, timotheegiet

Differential Revision: https://phabricator.kde.org/D23263