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/21.md

@@ -6,12 +6,13 @@ no: 21
 subtitle: "MentalSky – Personal Information Management, Networked Semantic Desktop"
 speaker: "Till Helmbold (HereToGo environment)"
 date: 2005-11-17T00:00:00+02:00
-event_date: 2005-11-17T20:00:00+02:00
-language: Deutsch
+event:
+  start: 2005-11-17T20:00:00+02:00
+  end:   2005-11-17T22:00:00+02:00
+language: de
 location: CCCB
 streaming: false
 ---
-{{< datengarten-infobox >}}
 
 Beschreibung
 ------------