docs: update examples + readme

examples/templates/example-advanced.html

@@ -1,4 +1,5 @@
 <h2>Path: {{ this | url(absolute=True) }}</h2>
 <p>This is: {{this}}</p>
 <p>Custom field, desc: "{{this.desc}}"</p>
+<p>Custom static, seen objects: {{this.custom_attr}}</p>
 <p>Children: {{this.children.all()}}</p>