Document the deprecation of the group command

In 3.4, we plan to deprecate the group command. New applications are expected to prefer the aggregation framework or mapReduce. Use of the group command in 3.4 will result in a warning being logged with the dochub link http://dochub.mongodb.org/core/group-command-deprecation. The following documentation changes are required:

• The 3.4 release notes should note the deprecation.
• The 3.4 manual should note that the group command is deprecated since 3.4.0. The deprecation notice should be added to both the command docs (https://docs.mongodb.com/manual/reference/command/group/) and the shell helper docs (https://docs.mongodb.com/manual/reference/method/db.collection.group/).
• The dochub link should direct users to a page which indicates that aggregation or mapReduce can be used in place of the group command.