feat: add publishDate to publish future publications now (#1025)

Similar to the talk template.

When `publishDate` is used in conjunction with `date`, it enables publishing future publications now.

See Hugo docs.

archetypes/publication/index.md

@@ -1,7 +1,12 @@
 +++
 title = "{{ replace .Name "-" " " | title }}"
+
+# Publication date.
 date = {{ .Date }}
 
+# Schedule page publish date.
+publishDate = {{ .Date }}
+
 # Authors. Comma separated list, e.g. `["Bob Smith", "David Jones"]`.
 authors = []