[topic] cargo dependencies: `features = ...`

[ultrasaurus]

It would be great to have a page that explains each feature group, ideally with an example for each feature group.

suggestion: /topics/feature-groups.md

It would elaborate on (and link from) this page: https://tokio.rs/docs/getting-started/cargo-dependencies/ (or if it makes sense could move some/all of that content into a topic page and link from prior page).