Point 'Getting started' link to docs.scala-lang.org, closes #774

Author: Philippus

_data/footer.yml

@@ -2,7 +2,7 @@
   class: documentation
   links:
     - title: Getting Started
-      url: "/documentation/getting-started.html"
+      url: "https://docs.scala-lang.org/getting-started.html"
     - title: API
       url: "https://www.scala-lang.org/api/current/index.html"
     - title: Overviews/Guides

index.md

@@ -12,7 +12,7 @@ gettingStarted:
   mainTitle: "Download"
   mainUrl: "/download"
   subtitle: "Getting Started"
-  subtitleLink: "/documentation/getting-started.html"
+  subtitleLink: "https://docs.scala-lang.org/getting-started.html"
   links:
     - title: "Milestones, nightlies, etc."
       url: "/download/index.html#other-releases"