Add explanatory text on the index and per-site pages

[webknjaz]

[...]
I totally missed that clicking on those links would take you to something like https://webknjaz.github.io/intersphinx-untangled/setuptools.rtfd.io/; I just assumed they linked to the named sites. I think adding some explanatory text on the index would make it even more useful. 😅

Originally posted by @bhrutledge in pypa/packaging.python.org#908 (comment)