plausible shortcode

in order to decouple header1.html and header2.html from the branding repository. Currently this is the only point for keeping these large headers in the branding repo.

• create shortcode {{ plausible domain }}
• adapt every page that needs to include this shortcode
• adapt header1.html and header2.html in pontifex-hugo
• delete header1.html and header2.html in pontifex-brand
• add customized shortcode {{plausible domain}} to pontifex-brand