Move reply by email docs to a new location

What does this MR do?

Move reply by email docs to a new location.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
• If working on CE, submit an MR to EE with the changes as well.

Marked the task Make sure the old link is not removed and has its contents replaced with a link to the new location. as completed

Marked the task Make sure internal links pointing to the document in question are not broken. as completed

Marked the task Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory. as completed

Marked the task If working on CE, submit an MR to EE with the changes as well. as completed

Status changed to merged

Mentioned in commit 38978fa5

Mentioned in issue #3349 (moved)