[Cards] Take current doc version from ReadTheDocs if available (#2934)

Author: mnocon

main.py

@@ -49,7 +49,9 @@ def cards(pages, columns=1, style="cards", force_version=False):
         canonical = current_page.canonical_url
         url_parts = re.search("//([^/]+)/([^/]+)/([^/]+)/", canonical)
         (site, language, version) = url_parts.groups()
+
         version = force_version or version
+        version = os.getenv("READTHEDOCS_VERSION_NAME", version)
 
         if isinstance(pages, str):
             pages = [pages]