Docs: add "new issue by email" to doc
This doc gathers all methods to add new issues to GL:
https://docs.gitlab.com/ee/user/project/issues/create_new_issue.html
But the feature "new issue by email" is missing here.
Also, when done, we'd better link it from https://docs.gitlab.com/ee/administration/reply_by_email.html
cc/ @victorwu
Edited by Marcia Ramos