Use absolute URLs for images

Many images appear broken on the Hugo site, as well as the production site if you happen to land on a page without a trailing slash.

See also:

• gitlab-org/gitlab-docs#660 (closed)
• gitlab-org/gitlab-docs#1770 (closed)

References:

• https://discourse.gohugo.io/t/image-path-with-relative-urls/21970/5
• https://discourse.gohugo.io/t/images-alongside-content/21649/2
• https://github.com/gohugoio/hugo/issues/6545

We can avoid this by rewriting image paths to use absolute URLs.