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

archetypes/datengarten.md

@@ -0,0 +1,22 @@
+---
+categories: ["Datengarten"]
+tags: []
+series: "Datengarten"
+title: "Datengarten {{ .File.TranslationBaseName }}"
+no: {{ .File.TranslationBaseName }}
+# Actual title goes here...
+subtitle:
+speaker:
+# Twitter, homepage, mail (use mailto:foo@example.com)
+#speaker_url:
+date: {{ .Date }}
+event:
+  - start: {{ .Date }}
+  - end:   {{ .Date }}
+location: CCCB
+language: Englisch
+streaming: true
+#recording: https://media.ccc.de/v/dg-{{ .File.TranslationBaseName }}
+---
+
+<Abstract goes here>