Define ogp_site_url for social media cards (#252)

Author: AA-Turner

Diff for: build_docs.py

@@ -751,9 +751,15 @@ def build(self):
         blurb = self.venv / "bin" / "blurb"
 
         if self.includes_html:
+            site_url = self.version.url
+            if self.language.tag != "en":
+                site_url += f"{self.language.tag}/"
             # Define a tag to enable opengraph socialcards previews
             # (used in Doc/conf.py and requires matplotlib)
-            sphinxopts.append("-t create-social-cards")
+            sphinxopts += (
+                "-t create-social-cards",
+                f"-D ogp_site_url={site_url}",
+            )
 
             # Disable CPython switchers, we handle them now:
             run(