feat!: move every layout to demo folder

- source/screens is now free for user files
- rename demo templates to their equivalent sources
- add demo maintenance template
- demo templates use html base element

source/screens/demo/index.json

@@ -0,0 +1,76 @@
+{
+  "index": [
+    {
+      "href": "index.html",
+      "text": "Index"
+    },
+    {
+      "href": "introduction.html",
+      "text": "Introduction"
+    },
+    {
+      "href": "basics.html",
+      "text": "Basics"
+    },
+    {
+      "href": "components.html",
+      "text": "Components"
+    },
+    {
+      "href": "layouts.html",
+      "text": "Layouts"
+    }
+  ],
+  "pages": [
+    {
+      "href": "pages/blank.html",
+      "text": "Blank"
+    },
+    {
+      "href": "pages/maintenance.html",
+      "text": "Maintenance"
+    },
+    {
+      "href": "pages/error/304.html",
+      "text": "304"
+    },
+    {
+      "href": "pages/error/400.html",
+      "text": "400"
+    },
+    {
+      "href": "pages/error/401.html",
+      "text": "401"
+    },
+    {
+      "href": "pages/error/403.html",
+      "text": "403"
+    },
+    {
+      "href": "pages/error/404.html",
+      "text": "404"
+    },
+    {
+      "href": "pages/error/408.html",
+      "text": "408"
+    },
+    {
+      "href": "pages/error/500.html",
+      "text": "500"
+    }
+  ],
+  "demo": [
+    {
+      "href": "examples/card.html",
+      "text": "Card"
+    },
+    {
+      "href": "examples/portal.html",
+      "text": "Portal"
+    },
+    {
+      "href": "examples/songbook.html",
+      "text": "Songbook"
+    }
+  ]
+}