ECE redirects - SAML sign outgoing messages

[eedugon]

Adds https://www.elastic.co/guide/en/cloud-enterprise/current/ece-sign-outgoing-saml-message.html to the appropriate doc as a mapped_page.

This is part of the work for https://github.com/elastic/docs-projects/issues/494

In a parallel activity we will request the redirect of that page to https://www.elastic.co/docs/deploy-manage/users-roles/cluster-or-deployment-auth/saml#_configuring_es_for_signing

In this case I think the problem was caused by a typo (_ instead of -). We already had this redirection and mapped_page configured, which is probably wrong:
https://www.elastic.co/guide/en/cloud-enterprise/current/ece_sign_outgoing_saml_message.html

I believe there's a typo and we could remove the redirection and the mapped_page of the underscored case. But I won't be doing it until I get confirmation from @colleenmcginnis or @shainaraskas .

@shainaraskas : I have also added a small update in the SAML doc as all the links about custom bundles need to be separated between ECH and ECE, because the procedures are completely different. let me know your thoughts and if you want to change the wording.

[github-actions]

🔍 Preview links for changed docs

• deploy-manage/users-roles/cluster-or-deployment-auth/saml.md

[shainaraskas]

changes lgtm

[shainaraskas]

I believe there's a typo and we could remove the redirection and the mapped_page of the underscored case. But I won't be doing it until I get confirmation from @colleenmcginnis or @shainaraskas .

I'd leave the old URL because it was valid with underscores in the 3.7 ECE docs: https://www.elastic.co/guide/en/cloud-enterprise/3.7/ece_sign_outgoing_saml_message.html

not sure what happened with 3.8 but would leave it alone to be safe.