document dvc import and dvc get (previously pkg commands)

[efiop]

UPDATE: See description in iterative/dvc#2160 instead.

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OLD: Some introductory descriptions for DVC packages (from Discord):

You have project named awesome_project, which has awesome_data in it
pkg will allow you to "import" the project
dvc pkg import awesome_project://awesome_data (syntax may be incorrect)

1. Any DVC repo is a package. Outputs inside packages can be called artifacts (that we'll also be able to work with)
2. There will be a set of commands to import/export these packages across different repositories and from a repository to a file system (just a data artifact)
3. basic use cases include things like: dataset registry (reuse a single dataset in multiple project, while having a single point of responsibility for the dataset itself), easy mechanism to pull a data artifact w/o doing git clone + dvc pull data.dvc, etc
4. in the future (no need to focus on this for now) any set of DVC files will be considered as package, thus making pipelines reusable

[efiop]

• Note: also need to make sure to add a note that old import was moved to import-url.