Docs: consolidate Email docs
The problem
Email docs are everywhere:
- https://docs.gitlab.com/ee/security/user_email_confirmation.html#user-email-confirmation-at-sign-up
- https://docs.gitlab.com/ee/tools/email.html
- https://docs.gitlab.com/ee/customization/branded_page_and_email_header.html
-
doc/user/admin_area/settings/email.md
(w/ https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5031)
The solution
- Create an index (or add to an existing one) for "Outgoing email" and "Incoming email"
- Consolidate the email docs - move them to correct files and folders, crosslink them all
cc/ @axil
Edited by Marcia Ramos