Introduce common incoming email setup doc and new issue by email doc

What does this MR do?

Introduce common incoming email setup doc and new issue by email doc

Review Apps: http://ee-docs-incoming-email.178.62.207.141.xip.io/ee/administration/incoming_email.html

TODO

• Port to CE https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17481
• Verify the instruction is correct, especially for the prerequisite part

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/41046, https://gitlab.com/gitlab-org/gitlab-ce/issues/24849