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/include.html

@@ -0,0 +1,20 @@
+{{ $path := "" }}
+{{ $markdown := false }}
+{{ if .IsNamedParams }}
+    {{ $path = .Get "path" }}
+    {{ with .Get "markdown" }}
+        {{ $markdown = . }}
+    {{ end }}
+{{ else }}
+    {{ $path = .Get 0 }}
+{{ end }}
+
+{{ with ($path | readFile) }}
+    {{ if $markdown }}
+        {{ $path | readFile | $.Page.RenderString (dict "display" "block") }}
+    {{ else }}
+        {{ $path | readFile }}
+    {{ end }}
+{{ else }}
+    {{ errorf "file not found: %s" $path }}
+{{ end }}