[WIP] dvc.yaml: rename stages -> run

[efiop]

Run is a more natural terminology for data scientists than stages so
it makes sense to rename it so it is more in-sync with it as well as our
commands (e.g. dvc run).

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[dmpetrov]

It is great idea to get rid of "stages", @efiop!
As Ivan said this might be confusion between a stage definition and a stage execution/instance. It might be better to use the term pipeline or stages. Pipeline is the best term I think.

[efiop]

@dmpetrov pipeline or pipelines? 🙂

[efiop]

Changed the PR to use pipeline, but I no longer like it myself. run made more sense to me. But at the same time I don't have an extremely strong opinion about it, as long as it makes sense to our target audience.

[skshetry]

@efiop, pipeline is not technically correct term to use here. Same with run. I think, stages is better than pipeline(s) or run, unless we have good alternatives.

[efiop]

Ok, agreed not to rename it. stages is the best one.