Move template files based on Hugo v0.146 revamp

See https://gohugo.io/templates/new-templatesystem-overview/

Under layouts/
- mv _default/* .
- mv partials _partials
- mv shortcodes _shortcodes
- mv taxonomy/list.html taxonomy.html
- mv taxonomy/term.html term.html

layouts/_partials/description-or-summary.html

@@ -0,0 +1,7 @@
+{{ with .Description }}
+    <p>{{ . | markdownify }}</p>
+{{ else }}
+    {{ with .Summary }}
+        {{ . }}
+    {{ end }}
+{{ end }}