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/single.markdown.md

@@ -0,0 +1,9 @@
+{{ with .File -}}
+  {{ with (.Path | readFile) -}}
+    {{ . -}}
+  {{ else -}}
+    {{ errorf "file not found: %s" .Path -}}
+  {{ end -}}
+{{ else -}}
+  {{ errorf "no file available for markdown template?" -}}
+{{ end -}}