Created entry file switch

Added checks for index.njk and data.json files to switch from demo file 
usage.
To prevent override of default files.

Updated explanation on demo entry page.

source/templates/data.json

@@ -0,0 +1,26 @@
+{
+  "demoadditionallinks": [
+    {
+      "href": "/demo.html",
+      "text": "Index"
+    }
+  ],
+  "demolinks": [
+    {
+      "href": "demo/error/403.html",
+      "text": "403"
+    },
+    {
+      "href": "demo/error/400.html",
+      "text": "400"
+    },
+    {
+      "href": "demo/error/500.html",
+      "text": "500"
+    },
+    {
+      "href": "demo/os.html",
+      "text": "OS"
+    }
+  ]
+}