feat: use collections and add frontmatter

source/screens/demo/examples/portal.njk

@@ -1,6 +1,10 @@
-<!-- portal.page -->
+---
+title: Portal
+tags:
+  - example
+---
 {% set pageBase = "../" %}
-{% set pageId = "index" %}
+{% set pageId = page.fileSlug %}
 {% set bodyClass = "body_portal" %}
 
 {% extends "demo/_default.njk" %}
@@ -14,6 +18,7 @@
 {% endblock %}
 
 {% block body %}
+<!-- {{ page.fileSlug }}.page -->
   <div class="portal">
     {{ gate.list(
     'Tor mit Symbol und Liste',