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 :)
I don't remember. I can only imagine I was trying to avoid the special behavior that `_locale` has. For example, if you're handling locales, you should kind of use `$request->getLocale()` in your controller to get it I think (in case it's being set somewhere other than your route).

In common practice, I don't think that's a big issue. I think we should keep your new changes. But, maybe we can add a quick note somewhere with a link to learn more about this special `_locale`. What do you think?