Overview: document how Tokio fits in with the rest of the async ecosystem

[nrc]

Ideally this would be in the overview chapter, but it could fit in 'getting started' or as some kind of appendix.

I think it would be good to mention where Tokio sits in the wider context, in particular: the futures crate, the futures bits and pieces in libstd, async/await in the language, mio, and other related crates.

Not sure if this is related or a separate thing, but I think it would be good to clarify to what extent "io" in these docs means network io - can tokio be used for non-network io? How common is that?