docs: simplify run examples, leaving bare min. usage

[jorgeorpinel]

Extracted from https://peckofpickledpojos.wordpress.com/2020/08/09/learning-dvc-by-trial-and-error:

the way stages are run assumes that the script src/prepare.py takes in command line arguments [incorrect but all of our examples look this way]... took me a few seconds before I could parse out what was happening

For example the first run example in https://dvc.org/doc/start/data-pipelines could just use python src/prepare.py as command and avoid repeating the dependency to avoid confusion. Probably other simplifications can be done to show the gist of run there and in other very-first examples of run users might find.

[jorgeorpinel]

Actually https://github.com/iterative/example-get-started/blob/master/src/prepare.py#L20 does take an argument from terminal.

Should we close this @shcheklein? Or change the example project so we don't have to repeat dependencies in run?

[jorgeorpinel]

Closed by #2075.