Get Started: structure and refactoring (without DVC 1.0)

[jorgeorpinel]

Branch rename from #1074

Should change PR target to master branch when #1051 is merged.

• Addresses part of create a dedicated Home page for docs at dvc.org/doc (design part left) #1073 (put docs Home in /doc)
• too many sections. maybe split/combine into 3? sections (basics, pipelines, experiment mgmt)
• "the biggest potion of this is content - examples, preparing repos, writing text, etc - more than moving sections around" (from get-started: initial refactoring #1051 (comment))
• Per Get rid of CSV/TSV metrics #1097 (comment):

  ideally we should update the getting started and remove all the raw metrics and xpaths (expect metrics show).

• Per Add params #1128 (comment):

  (dvc param) should become part of the Experiments section of the new Get started tutorial

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• Update for DVC 1.0! See user guide: DVC Files and Directories - 1.0 updates #1370

[jorgeorpinel]

A pyrrhic victory, but still I'm really glad this is merged so other PRs can move ahead. We need to solve some conflicts in them now though.