Do not apply epigraph style to all block quotes

Correct indentation where appropriate for stanzas that should not be
rendered as `<blockquote>` HTML elements.

Closes GH-10686

doc/_themes/sphinx13/static/sphinx13.css

@@ -273,7 +273,7 @@ div.quotebar {
     margin-left: 1em;
 }
 
-blockquote {
+blockquote.epigraph {
     font-size: 1.5em;
     padding-left: 1rem;
     margin-left: 0;