Add gendoc command

Using the new, great auto-doc in Cobra.

This commit also contains a current result of that command added to a commands section in docs.

Fixes #1136

docs/layouts/section/commands.html

@@ -0,0 +1,9 @@
+{{ partial "header.html" . }}
+<h1>Hugo Commands</h1>
+<p>This is an autogenerated and up-to-date (thanks to <a href="https://github.com/spf13/cobra">Cobra</a>) documentation for all the CLI commands in Hugo.</p>
+<ul>
+{{ range .Data.Pages }}
+<li><a href="{{ .URL }}">{{ .Title }}</a></li>
+{{ end }}
+</ul>
+{{ partial "footer.html" . }}