Stop rendering CS modules and room version fragments as standalone pages (#1317)

This is actually doing two things:

 * creating `{fragments,modules}/index.md` turns the fragments and modules into
   page resources, rather than pages in their own right. We have to update the
   shortcodes to match.

 * adding `headless: true` means that we don't render the pages.

The net effect is that we don't render pages like
https://spec.matrix.org/v1.4/rooms/fragments/v1-auth-rules/ and
https://spec.matrix.org/v1.4/client-server-api/modules/account_data/.

layouts/shortcodes/cs-module.html

@@ -9,5 +9,8 @@
 
 {{ $name := .Params.name }}
 
-{{ $page := .Site.GetPage (path.Join .Page.File.Dir "modules" (printf "%s%s" $name ".md"))}}
-{{ $page.Content }}
+{{ with .Site.GetPage "client-server-api/modules" }}
+    {{ with .Resources.GetMatch (printf "%s%s" $name ".md") }}
+{{ .Content }}
+    {{ end }}
+{{ end }}