Add new front matter element: excerpt (#227)

* This commit adds the excerpt front matter element, to show an
abbreviated version of the definition in social previews. This
also includes the documentation updates to support this change.

* This commit adds @olvb's suggestion to use 'set' to define the
alert & preview combo, to avoid repetitive code

* This commit updates the preview text to only include the flag if
it's "avoid" – if the flag is "warning" or there is no flag, it
will not appear at the beginning of the preview text.

Co-authored-by: Kathryn Grayson Nanz <kathryn@Kathryns-MacBook-Air.local>
Co-authored-by: Oscar <ovlb@users.noreply.github.com>
Co-authored-by: Kathryn <>

11ty/definitions/-misia.md

@@ -3,6 +3,7 @@ title: -misia
 slug: -misia
 speech: noun
 defined: true
+excerpt: from Greek for hate or hatred
 sub_terms:
   - text: Fat
     full_title: fatmisia