Create markdown output template

Further progress is blocked by Hugo feature requests:

- https://discourse.gohugo.io/t/render-alternative-output-format-outside-of-leaf-bundle-subdirectory/46968
- https://github.com/gohugoio/hugo/issues/8927

hugo.yaml

@@ -12,6 +12,11 @@ taxonomies:
   author: authors
   series: series
 
+outputs:
+  page:
+    - html
+    - markdown
+
 privacy:
   googleAnalytics:
     disable: true