readded hugo-book theme files.

themes/hugo-book/layouts/_markup/render-blockquote.html

@@ -0,0 +1,9 @@
+{{- $attributes := .Attributes -}}
+{{- $class := printf "book-hint %s" ($.AlertType | lower) -}}
+{{- if isset $attributes "class" -}}
+  {{- $class = printf "%s %s" $class (index $attributes "class") -}}
+{{ end -}}
+{{- $attributes = merge $attributes (dict "class" $class) -}}
+<blockquote {{ range $key, $value := $attributes }}{{printf "%s='%s'" $key $value | safeHTMLAttr}}{{ end }}>
+{{ .Text -}}
+</blockquote>