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/series.html

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