don't rm the docs until the build is successful

Author: RussTedrake

book/update.cgi

@@ -18,8 +18,8 @@ chdir "..";
 my $status = system('/bin/bash', '-c', '
     source venv/bin/activate &&
     poetry install --only docs &&
-    rm -rf book/python &&
     sphinx-build -M html underactuated /tmp/underactuated_doc &&
+    rm -rf book/python &&
     cp -r /tmp/underactuated_doc/html book/python
 ');