html extensions

Added some explicit links to .html where appropriate. In both the public DESY test site and the corresponding review app this seems to work.

_data/navigation.yml

@@ -24,20 +24,20 @@ header:
           url: "roadmap/"
 
         - title: Partners
-          url: "partners.md"
+          url: "partners.html"
 
         - title: Team
-          url: "team.md"
+          url: "team.html"
 
         - title: Jobs
           url: "https://www.helmholtz.de/en/jobs_talent/job_vacancies/"
 
 footer:
          - title: Contact
-           url: "contact.md"
+           url: "contact.html"
 
          - title: FAQ
-           url: "faq.md"
+           url: "faq.html"
            
          - title: Imprint
            url: "https://www.desy.de/imprint"