Add Meta description for Google Lighthouse

See also Track3#121

layouts/partials/structured-data.html

@@ -1,6 +1,7 @@
 {{/* We use some Hugo built-in templates, you can find their source here: */}}
 {{/* https://github.com/gohugoio/hugo/tree/master/tpl/tplimpl/embedded/templates */}}
 
+<meta name="description" content="{{ with .Description }}{{ . }}{{ else }}{{if .IsPage}}{{ .Summary }}{{ else }}{{ with .Site.Params.description }}{{ . }}{{ end }}{{ end }}{{ end -}}" />
 {{- template "_internal/schema.html" . }}
 {{- template "_internal/opengraph.html" . }}
 {{- template "_internal/twitter_cards.html" . }}