clarify external outputs section

[dmpetrov]

• First, https://dvc.org/doc/user-guide/external-outputs describes caches configuration, but not outputs. All the examples describe how to reconfigure cache. Let's rename it to something like Cache Reconfiguration or Setup Cache and let's change the section introduction paragraph accordingly.

• Local cache reconfiguration is missing. Something like dvc config cache.dir /mnt/cache.
  Also, it is not clear explain why we use cache.{s3, gs, ssh, hdfs} but for local cache we use cache.dir, not cache.local: cache.dir is a shortcut for cache.local.

• Consolidate everything related to Managing External Data (also including https://dvc.org/doc/user-guide/external-dependencies) under this document, adding an index.md file to do an overview of different approaches and features DVC gives to support it.

[jorgeorpinel]

Question:

Consolidate everything related to Managing External Data

What about external data sources? I.e. DVC remotes, dvc import and dvc import-url commands, or even more general concepts like "remote location" and "external data source". (from #566) Should we mention any of those in this new consolidated doc, or keep separate?

[shcheklein]

@jorgeorpinel I think we can definitely mention them to provide a full picture of different options how one can connect data from different sources

[jorgeorpinel]

This is a duplicate of #566 and #657 (comment). Please close 🙂