Rename commit cmd --no-commit options to cache and --no-cache

[jorgeorpinel]

After reviewing and simplifying the dvc commit cmd ref (in iterative/dvc.org/pull/1978) I realized it can be confusing to call this "committing" since we're mainly talking about caching (or not) data. Yes, it can also change hash values in DVC files but that's kind of an edge case given the main use for this: Data/code dev process where add/repro/run --no-commit are used until ready, and finished by a commit — the hash values are already saves in the DVC files by add/repro/run.

And we also have run --metrics/plots-no-cache, so no-cache is more consistent with those.

Finally, changing dvc commit to dvc cache and --no-commit to --no-cache would also help avoid confusions related to the Git analogies we use. Specifically, it will help clarify that DVC does not perform the actual versioning, but that Git does, through commits. Currently we need to have the following clarification near the top of the ref:

Note that this does not register project versions. That can be done with git commit.

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Maybe a change for 2.x? And it doesn't have to break backward compatibility if the old names are kept as aliases.

[skshetry]

Magic number 🎉