readded hugo-book theme files.

themes/hugo-book/layouts/_shortcodes/section.html

@@ -0,0 +1,13 @@
+{{- warnf "Section shortcode is deprecated and will be removed" -}}
+<dl>
+{{ range .Page.Pages }}
+  <dt>
+    <a href="{{ .RelPermalink }}">{{ partial "docs/title" . }}</a>
+  </dt>
+  {{ if (in $.Params "summary") -}}
+  <dd class="markdown-inner">
+    {{- or .Description .Summary site.Params.description | plainify | htmlUnescape }}
+  </dd>
+  {{ end -}}
+{{ end }}
+</dl>