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/list.html

@@ -0,0 +1,25 @@
+{{ define "main" }}
+    <h1>
+        {{ with .Params.icon }}{{ partial "icon.html" . }}{{ end }}
+        {{ .Title | markdownify }}
+        {{ partial "rss-link.html" . }}
+    </h1>
+
+    {{ with .Content }}
+        {{ . }}
+    {{ end }}
+
+    <div class="list-page-container">
+        {{ with .Sections }}
+            <div class="sections">
+                {{ range . }}
+                    {{ .Render "summary" }}
+                {{ end }}
+            </div>
+        {{ end }}
+
+        {{ partial "page-list.html" .Paginator.Pages }}
+    </div>
+
+    {{ partial "pagination.html" . }}
+{{ end }}