Button "Copy to clipboard" instead of "Add to description"

This allows the user to paste the code wherever they need. For example,
a blog post takes its contents from 'properties.content' and not from
the description field.

I also added an explanation for new attachment shortcode.

src/templates/nodes/edit_embed.pug

@@ -45,6 +45,10 @@
 				i.pi-plus
 				| Add New Attachment
 
+			p.text-muted
+				| Attachments can be included in any MarkDown field by using the #[code {attachment slug}] shortcode.
+				| This shortcode is placed on your copy-paste buffer by clicking "Copy to clipboard".
+
 		| {{ render_field(field, field.name) }}
 
 		| {% elif field.name == 'files' %}