use product email url

lib/docuseal.rb

@@ -3,6 +3,7 @@
 module Docuseal
   URL_CACHE = ActiveSupport::Cache::MemoryStore.new
   PRODUCT_URL = 'https://www.docuseal.com'
+  PRODUCT_EMAIL_URL = ENV.fetch('PRODUCT_EMAIL_URL', PRODUCT_URL)
   NEWSLETTER_URL = "#{PRODUCT_URL}/newsletters".freeze
   ENQUIRIES_URL = "#{PRODUCT_URL}/enquiries".freeze
   PRODUCT_NAME = 'DocuSeal'