[IMP] theme_notes: update the theme

This commit updates the theme to improve the design with new colors, new
pictures and new snippet customizations.

task-2590454

X-original-commit: 352692754c
Part-of: odoo/design-themes#501

theme_notes/static/description/index.html

@@ -7,7 +7,7 @@
         first projects on the web. Just drag & drop the new building blocks
         you need to easily build your layout. All layouts are made of sections,
         that you can easily combine to fit your specific project.</p>
-    <img src="Notes_description.png" class="img-fluid d-block mx-auto"/>
+    <img src="notes_description.jpg" class="img-fluid d-block mx-auto"/>
     <hr>
 
     <h2 class="text-muted">Notes Customization Tool</h2>