refactor(frontmatter): rename hide_date → show_date for consistent positive booleans

All other display toggles use positive naming (show_*, toc, share, etc.).
Flipping to show_date (default: true) aligns the API and reduces cognitive load.

BREAKING CHANGE

templates/documentation/config/_default/hugo.yaml

@@ -27,7 +27,7 @@ cascade:
       kind: page
     pager: true
     editable: true
-    hide_date: false
+    show_date: true
     show_date_updated: true
     reading_time: true
     commentable: true