Finish renaming command prompt to command line

I noticed a change made by @danchoi a few months ago to replace the term "command prompt" with "command line". This fixes the remaining instances.

(I am to blame for adding some of the "command prompt" -- I thought the terminology was weird but was editing a page that used it I wanted to be consistent with what was there!)

This also, instead of linking to the `command_prompt` page in this site, links to the docs site's command line page, which looks nicer and has been recently updated. I think it's good to only have the information in one place. Nothing links to `command_prompt` here now, so anyone merging this can delete it. If anything important in it hasn't already made it to the page in `docs`, I can look at that.