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-codeblock.html

@@ -0,0 +1,15 @@
+{{ $type := "text" }}
+{{ with .Type }}
+    {{ $type = . }}
+{{ end }}
+<div class="code-block">
+    <div class="code-header">
+        <pre class="code-type">{{ $type }}</pre>
+        <a href="javascript:void(0)" class="code-copy-button">
+            {{ i18n "copy_to_clipboard" }}
+        </a>
+    </div>
+
+    {{/* a div.highlight is already created by highlight function */}}
+    {{ highlight .Inner $type }}
+</div>

layouts/_markup/render-heading.html

@@ -0,0 +1,8 @@
+<h{{ .Level }} id="{{ .Anchor | safeURL }}" class="section-heading">
+    {{ .Text | safeHTML }}
+    <a class="heading-link" href="#{{ .Anchor | safeURL }}">
+        <span aria-label="{{ i18n "aria_heading_link" }}">
+            {{ partial "icon.html" "link" }}
+        </span>
+    </a>
+</h{{ .Level }}>

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. */ -}}