Update new page archetype

The theme provides an archetype of a new document to aid adding pages. This is
out of date and doesn't provide the front matter we expect. Modify it to
provide the minimum expected front matter.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

themes/apertis/archetypes/default.md

----
-subtitle: ""
-tags: []
----
++++
+title = ""
+weight = 100
+date = "2000-01-01"
++++