Tweaking Doctrine book chapter

The doctrine book chapter is a really long chapter, so long I haven't
finished it in my 2 years of sf docs experience ;-). I decided to put
some time in it to make it somewhat smaller.

I planned to do more significant changes, but I think that'll confuse
people. I already think these changes can confuse people, as they might
look for that documentation in the book chapter, while it was moved to
another article or even completely removed. But that might be a first
good testcase for our CHANGELOG plan :wink:

But I think it's good to invest time to make our book easy to read
again.

I've worked very precise with commits, in order to easy revert something
and in order to document why I did some changes. If you want some
clarification on some changes, you can read it in the long commit
description.

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | -