Revert "docs: don't suffix page permalink with a slash"

This breaks existing links with a trailing slash, so it's a no-go.

This reverts commit d1a7e030c5.

docs/_config.yml

@@ -4,7 +4,7 @@ title: systemd
 baseurl: "" # the subpath of your site, e.g. /blog/
 url: "https://systemd.io" # the base hostname & protocol for your site
 
-permalink: /:title
+permalink: /:title/
 
 # Build settings
 markdown: kramdown