Transfer Mission and HIFIS Structure

Transferred the descriptive stuff of HIFIS. Not beautiful but should contain the most important information. Second page/mission needs to be designed and discussed with AS,TH -> #11 (closed)

@frust45 can you please check how to make the link button to mission ("HIFIS MISSION") on the main page less dark? Or use different way to make this part clickable.

Closes #7 (closed)

_data/navigation.yml

@@ -18,7 +18,7 @@ header:
           url: "/"
 
         - title: Mission
-          url: "/"
+          url: "mission/"
 
         - title: Roadmap
           url: "/"