chore: Improve Helpcenter custom domains (#5456)

- Support rendering articles over frontend URL
- Support rendering articles over help center URL
- Support rendering help center home page in the custom domain root

app/controllers/public/api/v1/portals_controller.rb

@@ -1,13 +1,13 @@
 class Public::Api::V1::PortalsController < PublicController
   before_action :ensure_custom_domain_request, only: [:show]
-  before_action :set_portal
+  before_action :portal
   layout 'portal'
 
   def show; end
 
   private
 
-  def set_portal
-    @portal = @portals.find_by!(slug: params[:slug], archived: false)
+  def portal
+    @portal ||= Portal.find_by!(slug: params[:slug], archived: false)
   end
 end