docs: Copy some content of the upcoming 0.2 blog post to the docs

[tigerros]

Once https://github.com/marc2332/freya/pull/376 is finished, we should copy some of the things in the blog post over to the docs. For example, ActivableRoute and use_activable_route have lackluster docs, but the blog post goes into a lot more details + examples.

[marc2332]

sounds like a good idea 😄