Restructure the PKO reference page into a section

[jkodroff]

Problem

The Pulumi Kubernetes Operator (PKO) reference lives in a single ~888-line leaf page:
content/docs/integrations/clouds/kubernetes/pulumi-kubernetes-operator.md

It crams four different jobs into one wall of text: a getting-started tutorial, stack-authoring how-to, a feature reference, and an Argo CD guide that already has a fuller dedicated page at content/docs/iac/operations/continuous-delivery/argocd.md. It's hard to navigate, and there's no clean home for new operational/change-management content (see #16167).

Proposed restructuring

Convert the leaf page into a section: pulumi-kubernetes-operator.md → pulumi-kubernetes-operator/_index.md + child pages. The landing keeps the current URL, so top-level inbound links survive.

pulumi-kubernetes-operator/
├── _index.md            Overview, what PKO is, when to use it, "More information", links to child pages
├── installation.md      Install (Helm / Dev) + Service Account + Pulumi Cloud access + ESC config
├── defining-stacks.md   Create a Stack (Git repo / Flux source / Program object) + config values + structured config + env vars
├── stack-operations.md  Drift detection, state refresh, cleanup, prerequisites, external triggers, preview mode
└── argocd.md            Trim to a short stub that links to the existing CD guide (de-dupe)

Current section → new page mapping

Today (single page)	Moves to
Overview	_index.md
Install / Service Account / Cloud Access / ESC	installation.md
Create a Stack Resource (git / Flux / Program)	defining-stacks.md
Config values / structured config / env vars	defining-stacks.md
Drift / refresh / cleanup / prerequisites / triggers / preview mode	stack-operations.md
Use With Argo CD	collapse → link to continuous-delivery/argocd.md
More Information	fold into _index.md

Checklist / caveats

• Create the section directory + _index.md (preserve the current URL and all existing aliases).
• Split content into installation.md, defining-stacks.md, stack-operations.md per the mapping above.
• Anchor deep-links: existing links to #preview-mode, #stack-prerequisites, #drift-detection, etc. currently resolve on the single page. Splitting scatters them across new URLs — audit inbound anchor links (in-repo + known external) and add S3 redirects where needed (see move-doc anchor caveats).
• Argo CD de-dup: reconcile the reference page's Argo CD section against the dedicated iac/operations/continuous-delivery/argocd.md guide; keep the fuller version, stub the other.
• Nav: rewrite the integrations menu — the single kubernetes-clouds-operator leaf becomes a parent (_index.md) with child pages and per-page weights.
• Update internal links across content/docs/ that point at the old single page's anchors.
• make lint + make build pass; verify the landing URL and all aliases resolve.

Why this blocks #16167

#16167 asks for opinionated change-management guidance for PKO at scale (staged rollout, preview-before-apply). Those primitives (preview mode, prerequisites, resync) are currently scattered atomic blurbs with no coherent home. stack-operations.md is the seam that content hangs off of. This restructuring must land first.

/cc @jkodroff