Add ArgoCD integration documentation (#15202)

This adds documentation for using ArgoCD with the Pulumi Kubernetes Operator (PKO) to enable GitOps workflows for infrastructure as code.

Key additions:
- New dedicated ArgoCD documentation page with setup guides, advanced patterns, and best practices
- Integration examples showing multi-cluster deployments and environment provisioning
- Cross-references from PKO documentation and platform engineering solution pages
- ArgoCD logo added to CI/CD platforms grid on continuous delivery index page

The documentation shows how ArgoCD + PKO enables capabilities beyond traditional GitOps tools, such as provisioning entire Kubernetes clusters and deploying applications into them, while maintaining clean separation of concerns between GitOps orchestration and infrastructure provisioning.

Author: thoward

content/docs/iac/using-pulumi/continuous-delivery/_index.md

@@ -29,6 +29,10 @@ Pulumi can easily integrate into any continuous integration/continuous delivery
 > Looking to troubleshoot failures related to running Pulumi in CI/CD? Check out our [CI/CD troubleshooting guide](/docs/iac/using-pulumi/continuous-delivery/troubleshooting-guide).
 
 <div class="supported-cicd-platforms">
+    <a href="/docs/iac/using-pulumi/continuous-delivery/argocd">
+        <img src="/logos/tech/ci-cd/argo.svg" alt="ArgoCD">
+        <h4 class="no-anchor">ArgoCD</h4>
+    </a>
     <a href="/docs/iac/using-pulumi/continuous-delivery/aws-code-services">
         <img src="/logos/tech/ci-cd/aws-codedeploy.svg" alt="AWS Code Services">
         <h4 class="no-anchor">AWS Code Services</h4>