feat: use collections and add frontmatter

source/screens/demo/examples/card.njk

@@ -1,6 +1,10 @@
-<!-- card.page -->
+---
+title: Card
+tags:
+  - example
+---
 {% set pageBase = "../" %}
-{% set pageId = "card" %}
+{% set pageId = page.fileSlug %}
 {% set pageClass = "html_card" %}
 
 {% extends "demo/_default.njk" %}
@@ -13,6 +17,7 @@
 {% endblock %}
 
 {% block body %}
+<!-- {{ page.fileSlug }}.page -->
   <div class="card_bkg">
     <div id="dither"></div>
     <svg version="1.1" id="flag" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" width="100%" height="100%" viewbox="0 0 1920 1200" preserveaspectratio="xMinYMax slice">