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/24.md

@@ -5,13 +5,14 @@ title: "Datengarten 24"
 no: 24
 subtitle: "Retrocomputing oder: Wie waren Computer früher?"
 speaker: Dexter
-language: Deutsch
+language: de
 location: CCCB
 date: 2006-10-12T00:00:00+02:00
-event_date: 2006-10-12T20:00:00+02:00
+event:
+  start: 2006-10-12T20:00:00+02:00
+  end:   2006-10-12T22:00:00+02:00
 streaming: false
 ---
-{{< datengarten-infobox >}}
 
 Zusammenfassung
 ---------------