feat: add sitemap for help center (#9280)

* feat: start sitemap

* feat: add base url and last mod to sitemap

* fix: typo

* test: sitemap generation

* test: add draft articles

* fix: escape dots in regex matching

* feat: perpend protocol to the url

* feat: use ChatwootApp.help_center_root

* feat: don't parse the URL

* fix: function declaration

lib/chatwoot_app.rb

@@ -21,6 +21,10 @@ module ChatwootApp
     @custom ||= root.join('custom').exist?
   end
 
+  def self.help_center_root
+    ENV.fetch('HELPCENTER_URL', nil) || ENV.fetch('FRONTEND_URL', nil)
+  end
+
   def self.extensions
     if custom?
       %w[enterprise custom]