Sitemaps: Add wrapping `<div>` around sitemap in stylesheet.

This makes it easier to style the sitemap and for example center the entire content area.

Props ramiy.
Fixes #50622.
Built from https://develop.svn.wordpress.org/trunk@48471


git-svn-id: https://core.svn.wordpress.org/trunk@48240 1a063a9b-81f0-0310-95a4-ce76da25c4cd