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-link.html

@@ -0,0 +1,8 @@
+<a
+    href="{{ .Destination | safeURL }}"
+    {{ with .Title }}title="{{ . }}"{{ end }}
+    {{ if strings.HasPrefix .Destination "http" }}target="_blank" rel="noopener"{{ end }}
+>
+    {{- .Text | safeHTML -}}
+</a>
+{{- /* This comment gets rid of the trailing newline. */ -}}