Datasets cli improvements

[mariosasko]

This PR:

• replaces the code from the bug_report.md that was used to get relevant system info with a dedicated command (a more elegant approach than copy-pasting the code IMO)
• removes the download command (copied from the transformers repo?)
• adds missing help messages to the cli commands

[mariosasko]

Additionally, I've deleted the points that are not very relevant for this repo (I guess the deleted points originate from the transformers repo). With this change, running datasets-cli is identical to copy-pasting the code from bug_report.md, but is more elegant because it doesn't require launching the REPL and copy-pasting the code.

[lhoestq]

Nice thanks !

You did right removing the download command, it indeed came form transformers.

[lhoestq]

Thanks :)