Refresh a lot of the copy

This includes some small and some larger(er) changes, which I'll call out specifically in a moment, but generally:

* Typos and wording (small)
* Bring some tech stuff up to date (small)
* Change a few things about the rails engineering job description (bigger)
I feel like this section might benefit from being more closely worded to our various "mindsets", but for lack of that, thought it would be good to indicate again that we don't have all the answers, and candidates can teach as much as learn from us.