docs(guides): add crossplane with workload-identity guide

[haarchri]

This PR adds docs for configuring Crossplane to pull packages from private cloud provider container registries using Kubernetes Workload Identity.

Users frequently encounter issues when trying to use Crossplane with private container registries in managed Kubernetes environments like EKS, AKS, and GKE. While Crossplane already supports workload identity authentication, the setup process and prerequisites are not well documented. This leads to confusion and issue requests, as seen in crossplane/crossplane#6137.

The guide provides step-by-step instructions for configuring workload identity with Crossplane across AWS EKS with IAM Roles for Service Accounts and ECR, Azure AKS with Azure Workload Identity and ACR, and Google Cloud GKE with GKE Workload Identity and Artifact Registry.

This guide helps users leverage Crossplane's existing workload identity support without requiring static credentials or imagePullSecrets.

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