external link issues in popovers (baseurl?)

[stevejpurves]

Describe the bug

context
When I do follow an external link on a popover for DirectiveSpec at the bottom of this page:

https://mystmd.org/guide/plugins

expectation
I expected the page on github to open.

bug
But instead i get a 404

problem
This is a problem as some external links appear to be broken, at least on the mystmd.org site if not more widely.

Reproduce the bug

1. go to https://mystmd.org/guide/plugins
2. scroll to bottom
3. hover over DirectiveSpec
4. click the external link icon

List your environment

https://mystmd.org