add a user guide page about the optimization (advanced mode)

[shcheklein]

In the recent DVC release we are changing the way we are linking DVC controlled files with the working space via iterative/dvc#1841, iterative/dvc#1914 and a potentially a few other PRs.

The motivation for this change could be found here: iterative/dvc#1599

This is a major change and affect the user workflow, cache settings, performance, etc. Sometimes is a way that can make users unhappy - since copying is happening.

Things we need to do here:

• we should write a user guide section about the links (some information can be taken from - the Data files protection dvc#1599);
• we need to write a section "advanced mode" or "performance optimization" which should be a brief guide on how to switch DVC into using hardlinks, symlinks, etc, what caveats ppl should expect (that it's recommended to use protected mode by default and dvc unprotect in those cases);
• we should put links to these sections wherever it's appropriate;
• we should update the config doc - introduce new flags;
• we should review and update existing mentions of the dvc unprotect;
• Update a Tracked File section itself should be updated;

[ghost]

Related: https://github.com/iterative/dvc/pull/1997/files#r283492357

[jorgeorpinel]

Thanks for merging #345 Ivan. I still have to double check the last 2 check boxes in this issue though. I'll close again when I do so if you don't mind.

UPDATE: Will address in #368

[shcheklein]

oops, Github closed it automatically :)