[Umbrella] Release tooling consolidation: Makefile cleanup, deprecate gen-kubectldocs, add copy targets, switch website to apimd

Goal

Tidy up the release-team experience after the gen-apidocs markdown backend
landed. The Makefile has accumulated drift, gen-kubectldocs is functionally
replaced by gen-compdocs but still present, and several copy targets are
missing. Each of these is small in isolation but coordinated together they
make the per-release workflow significantly cleaner.

Sub-tasks

1. Makefile cleanup PR (https://github.com/kubernetes-sigs/reference-docs/pull/449)

   • Move K8S_ROOT / K8S_RELEASE / K8S_WEBROOT checks into the recipes
     that need them (currently fail every make invocation)
   • Drop the duplicate K8S_ROOT check
   • Drop the unused CURDIR=$(shell pwd) line in updateapispec
   • Add copyapimd target (symmetric with copyapi) for the markdown
     output → content/en/docs/reference/kubernetes-api/
   • Refresh the all target's printed list of supported targets

2. Deprecate gen-kubectldocs (https://github.com/kubernetes-sigs/reference-docs/pull/450)

   • Same pattern as the gen-resourcesdocs deprecation in
     #445:
     add a deprecation banner to gen-kubectldocs/README.md
   • Add a WARNING: echo to cli, copycli, cleancli targets pointing
     users to gen-compdocs

3. Add copycomp family of targets (https://github.com/kubernetes-sigs/reference-docs/pull/452)

   • copycomp-core: kube-apiserver, kube-controller-manager, kube-scheduler,
     kube-proxy → content/en/docs/reference/command-line-tools-reference/
   • copycomp-kubeadm: kubeadm tree → content/en/docs/reference/setup-tools/kubeadm/generated/
   • copycomp-kubectl: kubectl tree → content/en/docs/reference/kubectl/generated/
   • Mirrors @tengqm's 3-PR component handoff flow.

4. Fix cross-group ByKind collision PR (https://github.com/kubernetes-sigs/reference-docs/pull/447)

   • Tracked at #446 with concrete v1.36 evidence (cross-group "Other API
     versions" leaks in HTML output)
   • Independent of other items; can ship in this same release cycle

5. Switch k/website API reference to apimd output (https://github.com/kubernetes/website/pull/55758)

   • Replace content/en/docs/reference/kubernetes-api/ (currently
     gen-resourcesdocs output, semantic-category folders) with apimd output
     (API-group folders)
   • Update ~79 hand-curated docs that link to old kubernetes-api/<old-cat>/...
     URLs to use new kubernetes-api/<api-group>/... URLs
   • Depends on item 1 landing first (make copyapimd)

6. Hugo shortcode-based output (#440)

   • Goal: emit shortcode-wrapped fields instead of HTML tables so k/website
     templates control rendering (mobile layout, different render modes, etc.)
   • Tracked separately at #440; blocked on @lmktfy's markdown sketch
   • Sequenced after items 1, 3, 5 land

To contribute

Comment on this issue showing interest and raise a PR for any unchecked item.