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/page-metadata/item/categories.html

@@ -0,0 +1,11 @@
+{{ with (.GetTerms "categories") }}
+    {{ $categories := slice }}
+    {{ range . }}
+        {{ $categories = $categories | append (printf `<a href="%s">%s</a>` .Permalink .LinkTitle) }}
+    {{ end }}
+
+    <div class="page-metadata-section categories" aria-label="{{ i18n "aria_post_meta_categories" }}">
+        {{ partial "icon.html" "folder" }}
+        {{ delimit $categories ", " | safeHTML }}
+    </div>
+{{ end }}