docs layout: Support nested docs folders

See #682

layouts/partials/docs_sidebar.html

@@ -1,8 +1,8 @@
 {{ $current_page := . }}
-{{ $menu_name := .Section }}
+{{ $menu_name := (path.Base (path.Split .Path).Dir) }}
 {{ with (index .Site.Menus $menu_name) }}
 {{ else }}
-{{ errorf "Please define menu items named `[menu.%s]` in your %s front matter or in `config.toml`." $menu_name $menu_name }}
+{{ errorf "Please define menu items named `[menu.%s]` in your %s front matter or define `[[menu.%s]]` in `config.toml`." $menu_name .Path $menu_name }}
 {{ end }}
 
 {{ if eq $.Site.Params.search.engine 1 }}