Add info about the new location of Integrations

[ci skip]

doc/user/project/integrations/builds_emails.md

-## Enabling build emails
+# Enabling build emails
 
 By enabling this service, you will be able to receive e-mail notifications about
 the result status of your builds.

doc/user/project/integrations/emails_on_push.md

-## Enabling emails on push
+# Enabling emails on push
 
 By enabling this service, you will be able to receive email notifications for
 every change that is pushed to your project.