add hiddenPage param

with this param set to true, every effort is made to make the page 'undiscoverable'

layouts/_default/_markup/render-link.html

@@ -0,0 +1 @@
+<a href="{{ .Destination | safeURL }}" {{ with .Title }}title="{{ . }}"{{ end }} {{ if strings.HasPrefix .Destination "http" | and (.Page.Params.hiddenPage | in (slice true "true")) }}target="_blank" rel="nofollow noopener noreferrer"{{ end }}>{{ .Text | safeHTML }}</a>