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-list.html

@@ -0,0 +1,13 @@
+<div class="page-list">
+    {{ with . }}
+        {{ range . }}
+            <article class="page">
+                {{ .Render "summary" }}
+            </article>
+        {{ end }}
+    {{ else }}
+        <div>
+            {{ i18n "no_posts" }}
+        </div>
+    {{ end }}
+</div>