Increase indent of figures

This carries some risk, but on balance, I think that it is a good
change.

We see a lot of documents that include sourcecode or ASCII art that
appears at the same indent level as the rest of the text.  This makes it
hard to visually distinguish figures.

This increases the indent for these, where possible.  It retains the
indentation flexbox hack that ensures that wider figures reduce the
margin.

The risk is that this will double the indent where people have manually
added one.  The text formatter of xml2rfc doesn't add an indent, so this
is also an inconsistency in appearance.  My hope is that xml2rfc will be
updated to match, with people learning not to add their own indentation
to documents.

ietf/static/css/document_html_txt.scss

@@ -334,7 +334,7 @@ div:is(.artwork, .sourcecode).alignRight {
   justify-content: end;
 }
 div:is(.artwork, .sourcecode)::before {
-  flex: 0 1 3ch;
+  flex: 0 1 6ch;
   content: "";
 }
 div:is(.artwork, .sourcecode).alignRight::before {