Establish new code structure

Documentation:
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/13859
* [ ] Cleanup book and link the proposal
* [ ] Delete various README.md's with compatibility warnings

Implementation:
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/13877
* [x] https://github.com/kubernetes/test-infra/pull/37429
* [x] https://github.com/kubernetes-sigs/cluster-api/pull/13896
* [x] CABPK/KCP/core: https://github.com/kubernetes-sigs/cluster-api/pull/13894

Implementation: (not v1.14)

* [ ] Get rid of `exp` / `controllers` / `util`, `version` , `feature` (=> `pkg/feature`?), `internal` (incl. move `internal/setup`)
* Decide if to move either to pkg/ or utils
* Figure out where core/webhooks/util should go
* Pre-req: Create utils module
* Open question: Structure within pkg
* pkg/ vs pkg/util, additionally sub-packages for structure (e.g. like Kubernetes, topology, in-place)
* [ ] Move more packages from pkg/* into reconcilers/
* [ ] KCP pkg/*.go should probably move into a package and not be below pkg/* directly
* [ ] Consider moving top-level files away
* [ ] Align CAPD (maybe after Docker* removal, should be less work)
* [ ] Cleanup Dockerfiles
* Do we need also OS, ARCH (CGO?) (in l.55)
* De-duplicate ARGS
* Ask Claude for more improvements
* [ ] Review the new structure / proposal again and decide next tasks (previous POC branch: sbueringer/pr-code-structure)
* [ ] Drop duplicate of conn/dial (currently in KCP and in-memory server)
* [ ] [@sbueringer] Cleanup of "Docker" occurrences: docs, folder structure, docker.yaml, infrastructure-docker => infrastructure-dev