Guide: intro edits

This is mostly organizational and simplifying. However, I also removed prose which I took to be too basic given that we expect this to be read by programmers. Comments welcome.

cc @steveklabnik 
I still (see #18648) consider it silly and possibly even exclusionary to graphical text editor users to use this contrived pseudo-shell syntax when you can just say "Open a new file called `main.rs` in your editor."