Draft: Add details on proposed AWS SSO with Okta IdP for GitLab AWS Realms.

Why is this change being made?

This change is to document processes to be followed as https://gitlab.com/gitlab-com/gl-security/zero-trust/okta/-/issues/218 is implemented to provide a scalable way to provide administrative access to the GitLab AWS organization.

• Proposing GitLab AWS Realms as the MECEFU reference for all AWS accounts related to the organization of GitLab AWS Master to avoid confusion with other "Infrastructure" related related resources.

@mjozenazemian @cmccarty @hasharma For review and additions that are relevant to the AWS organization work recently reassigned.

cc @jeffersonmartin @pkaldis @dawsmith

References gitlab-com/sandbox-cloud/issue-tracking#17.

Author Checklist

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