Rustc-dev-guide: Expand descriptions of the various compilation stages

[frewsxcv]

This is what we currently have: https://github.com/rust-lang-nursery/rustc-guide/blob/46864ca9714707f18e658afba9d2ea62afbe2374/src/how-to-build-and-run.md#running-xpy-and-building-a-stage1-compiler

There's a relevant thread on reddit about what these stages all mean and I think the descriptions in this answer could be incorporated into this section in the rustc guide

[ehsanmok]

I am interested in this as my very first PR. I'll prepare it over this weekend.

[mark-i-m]

Thanks @frewsxcv and @ehsanmok :)