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/summary.html

@@ -0,0 +1,21 @@
+<h2 class="page-title">
+    {{ with .Parent.Params.icon }}{{ partial "icon.html" . }}{{ end }}
+    <a class="heading-link" href="{{ .Permalink }}">{{ .Title | markdownify }}</a>
+    {{ if .Draft }}{{ partial "icon.html" "write" }}{{ end }}
+</h2>
+
+{{ partial "page-metadata/post-short.html" . }}
+
+{{ with .Resources.GetMatch "feature" }}
+    <div class="post-media">
+        {{ partial "figure.html" (dict "src" .) }}
+    </div>
+{{ end }}
+
+<div class="page-description">
+    {{ partial "description-or-summary.html" . }}
+</div>
+
+<div class="readmore">
+    <a href="{{ .Permalink }}">{{ i18n "read_more" }} &rarr;</a>
+</div>