[3.12] pythonGH-121970: Extract pydoc_topics into a new extension (pythonGH-131256)

(cherry picked from commit c1a02f9)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Author: AA-Turner

Doc/tools/extensions/pydoc_topics.py

@@ -143,7 +143,7 @@ def write_documents(self, _docnames: Set[str]) -> None:
                 visitor = TextTranslator(document, builder=self)
                 document.walkabout(visitor)
                 body = "\n".join(map(str.rstrip, visitor.body.splitlines()))
-                self.topics[topic_label] = body
+                self.topics[topic_label] = body + "\n"
 
     def finish(self) -> None:
         topics_repr = "\n".join(