add description tag

website/static/header.html

@@ -1,4 +1,8 @@
 <meta name="viewport" content="width=device-width, initial-scale=1" />
+<meta
+  name="description"
+  content="NiceGUI is an easy-to-use, Python-based UI framework, which shows up in your web browser. You can create buttons, dialogs, markdown, 3D scenes, plots and much more."
+/>
 
 <!-- https://realfavicongenerator.net/ -->
 <link rel="apple-touch-icon" sizes="180x180" href="favicon/apple-touch-icon.png" />