Add banner text to main.min.js (#880)

- Add `---` YAML Front Matter as a workaround to allow the theme gem's version to be overridden locally.
- Adjust page scope in Front Matter default to avoid adding a layout to `main.min.js`
- Fixes #874, Fixes #722

docs/_config.yml

@@ -195,7 +195,7 @@ defaults:
       related: true
   # _pages
   - scope:
-      path: ""
+      path: "_pages"
       type: pages
     values:
       layout: single