Restore existing email workaround page

[shainaraskas]

https://elasticco.atlassian.net/browse/IAM-860?linkSource=email

this page: https://web.archive.org/web/20250000000000*/https://www.elastic.co/guide/en/cloud/current/ec-getting-started-existing-email.html

resolves this bug: https://github.com/elastic/cloud/issues/93688#issuecomment-992556085

currently redirects here: https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/create-an-organization

consider putting it here:
https://www.elastic.co/docs/cloud-account/update-your-email-address

or somewhere in troubleshooting (there's no good place for it)