Resolve "Add Roadmap"

First roadmap added and linked, separated for 3 clusters + overall.

@SpickerHZB @andreasfk @frust45 can you possibly check the correctness and completeness of the roadmaps points for cloud+software clusters (at least for the time being)

@jandt-desy checks the same for 2 more backbone issues

@frust45 you said that color coding of the clusters should be doable. Can you please check that (if it's easy).

Closes #8 (closed)

_data/navigation.yml

@@ -21,7 +21,7 @@ header:
           url: "mission/"
 
         - title: Roadmap
-          url: "/"
+          url: "roadmap/"
 
         - title: International
           url: "/"