De-emphasise blank lines for attaching content to grandparent list items

[rockyallen]

It is currently the first method discussed, but it is fragile, goes against good source code formatting practice (IMO), may have bugs, and has a better alternative. Make the fenced block method the main one, and relegate blank lines to a sidebar. Or treat it as deprecated/python.

[mojavelinux]

I have applied this change to the new documentation. See https://docs.asciidoctor.org/asciidoc/latest/lists/continuation/#attaching-to-an-ancestor-list