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.
See merge request !6517
38978fa53f