Move out datengartens into their own section

- /datengarten/index.html returns a blog-like list
- /datengarten/index.ics returns a VCALENDAR
- /datengarten/index.xml returns a schedule.xml-style view

No more pretty rendering includes in the content pages,
some more cleanup in the front matter

content/datengarten/42.md

@@ -5,13 +5,14 @@ title: "Datengarten 42"
 no: 42
 subtitle: "Open Source Robotics"
 date: 2013-05-16T00:00:00+02:00
-event_date: 2013-05-16T20:00:00+02:00
+event:
+  start: 2013-05-16T20:00:00+02:00
+  end:   2013-05-16T22:00:00+02:00
 location: CCCB
 speaker: Bill Morris
-language: Englisch
+language: en
 streaming: false
 ---
-{{< datengarten-infobox >}}
 
 Am 16. Mai ab 20 Uhr wird uns Bill Morris von *I Heart Engineering*
 etwas über das *Robot Operating System* und über