adds shortcode for manual pages links

layouts/shortcodes/man.html

@@ -0,0 +1,5 @@
+{{- $page := .Get 0 | default "man" }}
+{{- $section := .Get 1 | default "1" }}
+
+<a href="https://www.man.page/{{ $section }}/{{ $page }}" target="_blank" rel="noreferrer">{{ $page }}({{ $section }})</a>
+