Add split off user email addresses API docs to global navigation

What does this MR do and why?

For: technical-writing#1075, and as a companion to gitlab!167111 (merged), add split off user email addresses API docs to global navigation.

• Before: https://docs.gitlab.com/ee/api/user_email_addresses.html
• After: https://eread-add-split-off-user-email-addresses-api-docs-to-global-nav.docs.gitlab-review.app/ee/api/user_email_addresses.html

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