Explicitly mention email OTP in relation to 2FA

What does this MR do and why?

⚠️ This MR needs Add Email OTP section fo user 2FA section (!215521 - merged) merged first.

This MR updates our documentation to make it more clear the impact of having email OTP enabled in the context of password based authentication.

Preview pages

• https://docs.gitlab.com/upstream-review-mr-ee-216074/security/two_factor_authentication#enforce-2fa-for-all-users-in-a-group
• https://docs.gitlab.com/upstream-review-mr-ee-216074/topics/git/clone#clone-with-https
• https://docs.gitlab.com/upstream-review-mr-ee-216074/user/packages/container_registry/authenticate_with_container_registry#authenticate-with-username-and-password

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/570175+

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