remove gratuitous aside

Author: mbostock

docs/system-guide.html

@@ -7,8 +7,6 @@
     <link rel="stylesheet" href="./style.css">
   </script>
   <script id="38" type="text/markdown">
-    <aside>This document is itself a notebook, and this paragraph is a Markdown cell.</aside>
-
     Notebooks are interactive documents comprised of small programs called _cells_. Cells can be written in JavaScript, Markdown, HTML, and other languages. Most logic is coded in JavaScript, while prose is typically written in Markdown.
   </script>
   <script id="41" type="module" pinned="">