Revert part of "mv pseudonyms and db split to separate "data separation" page"

This reverts parts of !78 (merged)

The securtiy chapter was meant as a overview for the different security protections in castellum. I got the feeling that the database split should be mentioned there and is too technical for the privacy chapter.

I restored the pre-!78 version of this section. This means that there are no a few redundancies between the privacy and security chapters. But I think this is ok given the overlap these topics inadvertently have.