Improve compiler top-level description in rustc-dev guide

### Location

https://rustc-dev-guide.rust-lang.org/overview.html

### Summary

@akarahdev wrote a very nice top-level overview [on reddit](https://www.reddit.com/r/rust/comments/13zdmvc/how_rust_transforms_into_machine_code/) which I think could be used to improve the existing (and a bit outdated) [overview of the compiler](https://rustc-dev-guide.rust-lang.org/overview.html).