feat: unify layout slot architecture - make header/footer configurable positions, footer as array, add defaultPosition fallback, add tests, update docs

quartz.config.default.yaml

@@ -205,6 +205,9 @@ plugins:
       links:
         GitHub: https://github.com/jackyzha0/quartz
         Discord Community: https://discord.gg/cRFFHYye7t
+    layout:
+      position: footer
+      priority: 50
   - source: "@quartz-community/recent-notes"
     enabled: false
   - source: "@quartz-community/spacer"
@@ -248,10 +251,6 @@ plugins:
       unlistWhenEncrypted: false
       outputPath: static/encryptedContentIndex.json
     order: 900
-    layout:
-      position: body
-      priority: 100
-      display: all
   - source: "@quartz-community/stacked-pages"
     enabled: false
     layout: