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/documentation/examples/basic.md

@@ -13,6 +13,7 @@ title: Example Word
 slug: 'example-word'
 speech: noun
 defined: true
+excerpt: This is a definition.
 ---
 
 This is a sentence. It describes a word. This definition is written in Markdown. So you can do stuff. [Including links](/link), for example.

11ty/documentation/examples/flagged-word.md

@@ -21,6 +21,7 @@ flag:
   text: 'Ableist Slur'
   level: avoid
 defined: true
+excerpt: mentally deranged; demented; insane.
 speech: noun
 alt_words:
   - abundant
@@ -65,6 +66,7 @@ This example is the definition of [performative allyship](/definitions/performat
 title: performative allyship
 slug: performative-allyship
 defined: true
+excerpt: when an individual or group of power/majority/privilege loudly profess(es) their actions in the name of 'allyship,' while actively conducting harm to the group they claim to support
 speech: noun
 flag:
   level: tool