Menu links should probably use absolute URL

[laeti-tia]

When landing on a non-existing page inside a subdirectory, there is no way to go back to the Homepage or any other page easily. All links in the navigation menu seem to be relative to the current directory, so from one 404 you go to another 404.

For example, we still have a redirect from http://downloads.perfsonar.net that points to https://www.perfsonar.net/about/where-can-it-be-downloaded/

And the redirect should be corrected too… ;-)

[mfeit-internet2]

This proved not as simple as it looked. It works fine when the 404 is on a top-level page (e.g., /foo.html), but anything further down (/foo/bar.html) doesn't render the 404 page as being in the right relative place.

Unfortuantely, Jekyll can only render the 404 page once and doesn't have one per level. I'm going to icebox this and revisit it another time.