Use the post's meta description if it exists, otherwise use the
one from the site's config. Set them using the "description" key
in frontmatter or site config.
Note: this means there is no way to NOT have a description. You
must have *at least* a site-wide description and *optionally* a
description for each page/post's front matter.
Partials are nice, but blocks are a better base construct. Right
now there is basically only layouts/_default/baseof.html that is
doing most of the work.
See: https://gohugo.io/templates/blocks