Update link to point to new location

What does this MR do and why?

The doc page doc/user/profile/unknown_sign_in_notification.md was removed and now redirects to doc/user/profile/notifications.md. This MR updates the UI to point to the new documentation page.

Related to !109709 (merged) and technical-writing#742 (closed).

How to set up and validate locally

You can validate the redirect by visiting https://docs.gitlab.com/ee/user/profile/unknown_sign_in_notification.html in your web browser.

MR acceptance checklist

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