clarify external outputs section

• 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.

[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

[efiop]

I think External outputs name is suitable since the article explains both cache configuration(as a necessity for outputs) and outputs themselves. You can see dvc run commands showing how to use external outputs.

We indeed need to explain better external output case for local file outside of the dvc project. Also, cache.dir is a shortcut for cache.local. That needs better explaining as well.