docs(design-proposals): revise ComputePlane — operator-owned module wrapping apps/kubernetes

Supersede the preset-field revision (#27): deliver ComputePlane as a
Cozystack-owned tenant module (packages/extra/computeplane) that deploys the
ordinary apps/kubernetes chart with operator-fixed values via a source-only
PackageSource re-declaration. No new CRD, no new controller, no fields on
kind: Kubernetes; the module registers its own thin ComputePlane module-kind.

Addressing the review: state the computeplane release name as a stable
external contract (renaming it would at best dangle every placement:
ComputePlane secretRef and at worst have Helm uninstall a live Kamaji
cluster), and ground the design's shape structurally — a second
ApplicationDefinition because AD properties are per-kind, plus a wrapper
chart because the AD carries no fixed-values facility — rather than in the
apps-vs-extra directory split, so the rationale composes with #39's
capability-field model tracked as a separate stream.

Assisted-By: Claude
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

Author: kvaps