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/seo-description.html

@@ -0,0 +1,9 @@
+{{- with .Description -}}
+    <p>{{- . | markdownify -}}</p>
+{{- else -}}
+    {{- with .Summary -}}
+        {{- . -}}
+    {{- else -}}
+        {{- .Site.Params.description -}}
+    {{- end -}}
+{{- end -}}