Re-organize changelog to make updating to 0.6 easier

[davidpdrsn]

trafficstars

For 0.6 I wanna re-organize to changelog by grouping the changes into areas (like routing, extractors, responses, etc) and provide examples for how to migrate from 0.5 to 0.6. This should also explicitly call out subtle routing changes like the ones made in https://github.com/tokio-rs/axum/pull/1086

[jplatte]

Maybe we can also highlight that axum recently had its first birthday? 👶🏼

[davidpdrsn]

Hehe sure!