add docs for editing the content

content/_data/program.md

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+# program.yaml
+
+The top level keys must be a *quoted date in yyyy-mm-dd format*.
+
+Each contains a list of events, which have the following properties:
+- `start`: time in hh:mm
+- `end`: time or null/omit if open ended
+- `title`, `description`, `location`, `annotation`: either de/en versions (like this [^1]) or simply a string if it should be the same for all languages.
+  - `description`, `location` and `annotation` can use some markdown, e.g. hyperlinks or italics
+  - `location` and `annotation` are optional.
+- `checkin_link`: optionally, a url where people need to register or can find more important information
+- `livestream`: true, if this event will be live streamed. Can be omitted otherwise.
+    
+[^1]: ```yaml
+        title:
+          de: Begrüßung
+          en: Grand Opening
+      ```
+      Both en and de versions must be set and non-empty.