docs: add subdir for experimental/core-landing-zone during kpt pkg get on both core-landing-zone packages - to avoid collision

adjust section 2-2 to handle the two core-landing-zone package downloads
https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit/tree/main/docs/landing-zone-v2#2-create-your-landing-zone

```
admin_@cloudshell:~/pdt-arg/main (pdt-arg-kcc11)$ kpt pkg get https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit.git/solutions/gatekeeper-policies@main
Package "gatekeeper-policies":
Fetching https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit@main
From https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit
* branch main -> FETCH_HEAD
+ 4ec915c...8f765e2 main -> origin/main (forced update)
Adding package "solutions/gatekeeper-policies".

admin_@cloudshell:~/pdt-arg/main (pdt-arg-kcc11)$ kpt pkg get https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit.git/solutions/core-landing-zone@main
Package "core-landing-zone":
Fetching https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit@main
From https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit
* branch main -> FETCH_HEAD
Adding package "solutions/core-landing-zone".

Fetched 1 package(s).
admin_@cloudshell:~/pdt-arg/main (pdt-arg-kcc11)$ ls
core-landing-zone core-landing-zone_old gatekeeper-policies gatekeeper-policies-old packages pubsec-declarative-toolkit setters.yaml

handle duplicate with experimentation - get into subdirectory to fix
admin_@cloudshell:~/pdt-arg/main (pdt-arg-kcc11)$ kpt pkg get https://github.com/GoogleCloudPlatform/pubsec-declarative-toolkit.git/solutions/experimentation/core-landing-zone@main
Error: destination directory "core-landing-zone" already exists

```

do a mkdir experimentation first