Update most important book articles to follow the best practices

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all (or 2.6?)
| Fixed ticket | lots of complaints :)

I've decided to do only the most important articles for the first PR, otherwise the diff will be too big to get a nice review. I'll do a seperate PR for the Page Creation article, since that needed quite so rewriting.

Btw, I also couldn't resist to fix other minor things :)
It's a bit weird that we first teach users to use routing annotations and here tell them that they commonly define them in, for example, YAML. I think we could better reword this paragraph a bit.