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/authors.html

@@ -0,0 +1,16 @@
+{{ with .Params.authors }}
+    <span class="page-metadata-item" aria-label="{{ i18n "aria_post_meta_authors" }}">
+        {{ partial "icon.html" "user-circle" }}
+
+        {{ if index $.Site.Taxonomies "authors" }}
+            {{ $authors := slice }}
+            {{ range . }}
+                {{ $url := (printf "authors/%s" (. | anchorize)) | absLangURL }}
+                {{ $authors = $authors | append (printf `<a href="%s">%s</a>` $url .) }}
+            {{ end }}
+            {{ delimit $authors ", " | safeHTML }}
+        {{ else }}
+            {{ delimit . ", " }}
+        {{ end }}
+    </span>
+{{ end }}