Add render-link layout for opening external URLs in a new tab

[ajschmidt8]

Hugo has Markdown render hooks that enable us to customize how links are rendered:

• https://gohugo.io/templates/render-hooks/

We should update the render-link.html template to ensure that external URLs are opened in a new tab.

These resources will be useful for the implementation:

• https://digitaldrummerj.me/hugo-links-to-other-pages/
• https://discourse.gohugo.io/t/simple-way-to-open-in-a-new-tab/28677

As part of this effort, we should be sure to also remove all of the {: target="_blank"} tags from Jekyll that are no longer valid in Hugo.