[WIP] Revisiting structure part 1

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | partly #523

During this Community Hack Day, I'll try to work on the revisited doc structure.
this (and the other 1 line intros) prove the point that the separation between bundle doc and config reference was not a good thing.

do we have these short descriptions in an overview, or on the index of each bundle doc? i think they are valuable (but of course not in the config reference section)