Define create-social-cards tag

Author: AA-Turner

build_docs.py

@@ -745,6 +745,9 @@ def build(self):
         blurb = self.venv / "bin" / "blurb"
 
         if self.includes_html:
+            # For opengraph previews (requires matplotlib)
+            sphinxopts.append("-t create-social-cards")
+
             # Disable CPython switchers, we handle them now:
             run(
                 ["sed", "-i"]