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'm totally being a pedant here, but this is the result of `find -name '*.rs' -type f -exec cat {} \; | sed -e 's/^.*\(.\)$/\1/' | sort | uniq -c | sort -rn` on `rust-lang/rust/src`:

https://gist.github.com/tomjakubowski/480a82279c145e3428a7

Clearly most lines of Rust code do not end with a `;`.