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/_markup/render-heading.html

@@ -0,0 +1,8 @@
+<h{{ .Level }} id="{{ .Anchor | safeURL }}" class="section-heading">
+    {{ .Text | safeHTML }}
+    <a class="heading-link" href="#{{ .Anchor | safeURL }}">
+        <span aria-label="{{ i18n "aria_heading_link" }}">
+            {{ partial "icon.html" "link" }}
+        </span>
+    </a>
+</h{{ .Level }}>