Drops external outputs and updates external data guides

[dberenbaum]

Part of #4513.

This drops external outputs but also makes other changes to the guides about external data:

• Reorganizes into a page in data management about importing external data, and another one in pipelines about external deps and outs.
• For importing external data, separates it from external dependencies into its own page and highlights how to avoid unnecessary downloads/uploads.
• For external deps and outs, drops external cache, focuses on no-cache external outputs, and mentions plans to address lack of caching with cloud versioning.

Edit: closes #520

[dberenbaum]

The entire external dependencies section is moved from https://dvc.org/doc/user-guide/data-management/importing-external-data#how-external-dependencies-work and otherwise is untouched except for taking out import examples since they are addressed in other guides and aren't directly relevant to pipeline dependencies.

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[dberenbaum]

@efiop I'm hesitant to merge this because it relies on keeping --outs-no-cache, so maybe we should take a look and make sure this makes sense to implement first.

[dberenbaum]

@efiop Still need to update the ref and other areas to drop --external everywhere.

[dberenbaum]

@efiop Still need to update the ref and other areas to drop --external everywhere.

Done. PTAL 🙏.