Add guidelines for setting up Kubernetes cluster (minikube, kind, kubeadm) in Quickstart documentation

What would you like to be added:

Add setup guidance for Kubernetes prerequisites in the Quickstart documentation.

The current prerequisites section mentions that users need a running Kubernetes cluster and kubectl configured to connect to it. However, there are no setup references or onboarding instructions for users who are new to Kubernetes and want to run PipeCD locally for evaluation or development purposes.

Suggested additions:

• Brief references to local Kubernetes options such as Minikube, kind, and Docker Desktop Kubernetes
• Minimal verification commands such as:
  • kubectl cluster-info
  • kubectl get nodes
• An optional minimal local setup example using Minikube

Example:

brew install minikube
minikube start

Why is this needed:

New users following the Quickstart guide can get blocked immediately if they do not already have a Kubernetes cluster configured.

This commonly results in errors such as:

The connection to the server localhost:8080 was refused

before the actual PipeCD setup even begins.

Adding a small setup/reference section would improve the onboarding experience for first-time users trying PipeCD locally and reduce confusion around the prerequisites stage.