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 :)
+1 to including annotation examples, and as the first tab. And although I want to put annotations at the main path, I *do* want to put in a little bit of effort to highlight a bit that YAML is also a common method (just because some percentage hate annotations, so I don't want someone to think that we're forcing this on you).

Is this section relevant? I'd have to read it in context, but I think it still is (though probably less important, so it might be further down or even in another entry).