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/pages/error/500.njk

@@ -1,24 +0,0 @@
-<!-- error-500.page -->
-{% set pageBase = "../../" %}
-{% set pageId = "500" %}
-{% set bodyClass = "body_status" %}
-
-{% extends "hippie/_maintenance.njk" %}
-
-{% block title %}{{ pageId }}{% endblock %}
-{% block head %}
-{{ super() }}
-{% endblock %}
-
-{% block main %}
-<main class="main_site">
-  <h1>{{ pageId }}</h1>
-  <h3>Server-Fehler</h3>
-  <p>Allgemeiner Server Fehler!!! <dfn>Internal Server Error</dfn></p>
-  <blockquote cite="https://de.wikipedia.org/wiki/HTTP-Statuscode#5xx_%E2%80%93_Server-Fehler">
-    <p>Dies ist ein „Sammel-Statuscode“ für unerwartete Serverfehler.</p>
-    <p class="quote_source"><a href="https://de.wikipedia.org/wiki/HTTP-Statuscode#5xx_%E2%80%93_Server-Fehler">Wikipedia</a></p>
-  </blockquote>
-</main>
-{{ super() }}
-{% endblock %}