cmd ref: advanced examples for import (and get)

[jorgeorpinel]

The previous PR was too long. We're extracting this part, which is pending and hopefully the last related to dvc import and dvc get. See #464 (review)

• Note that the import-url ref and external-dependencies guide also needs an update (See Write import, get, and get-url refs; std. existing refs; other doc format/lang. updates #464 (review)).

• Create more sample repos for advanced import and possibly get command example/s. See cmd ref: advanced examples for import (and get) #487 (comment) below.

• Actually create/update advanced examples for import/get commands.

• Check the example-pipeline carefully. It looks like it might be broken now. (Check both examples in fact.) Moved to reorganize get-started examples into tutorials #564

• Lets also update get started, work on use cases, etc. to incorporate these important new commands (include also dvc update).

[shcheklein]

Cool. Makes sense. Not the last though - we'll need to update get started, work on use cases, etc. :)

[jorgeorpinel]

More details/ideas on " Lets also update get started, work on use cases, etc. to incorporate these important new commands, as well as dvc update" (from this Discord thread):

"...we can create a repo that serves as a dataset registry and update get started carefully to show how that dataset can be reused" including:

• The raw data file we wget in https://dvc.org/doc/get-started/add-files
• To show some dataset evolution we can use images from https://dvc.org/doc/get-started/example-versioning (https://data.dvc.org/tutorial/ver/data.zip and https://data.dvc.org/tutorial/ver/new-labels.zip)
• Downloads in https://dvc.org/doc/get-started/example-pipeline
• https://medium.com/@fullstackml/aab8162f8e3f (https://github.com/iterative/cnn_tutorial)?

See #564 related to some of the above, may have to address them there first.

Let's do it the same way as https://github.com/iterative/example-get-started-dev – a script that generates this registry. UPDATE: We decided to not it that way. See