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/_shortcodes/see-also.html

@@ -0,0 +1,7 @@
+<div class="see-also">
+    {{ with .Site.GetPage (.Get 0) }}
+        <p>{{ i18n "see_also" }} <a href="{{ .Permalink }}">{{ .Title | markdownify }}</a></p>
+    {{ else }}
+        {{ errorf "Page could not be found for see-also shortcode: %s" .Position }}
+    {{ end }}
+</div>