doc: how to use the SDK leader election with new layout

We speak over it in the Migration guide to +1.0.0 but we need to have a doc for ti with details steps and just link in the migration guide. See: https://master.sdk.operatorframework.io/docs/building-operators/golang/project_migration_guide/#migrate-maingo.

Steps required to make it work on the cluster

• Import the lib

go get -u github.com/operator-framework/operator-lib

• Update main.go which will be as:

ctx := context.TODO()
	// Become the leader before proceeding
	err := leader.Become(ctx, "anyproject-lock")
	if err != nil {
		os.Exit(1)
	}

	mgr, err := ctrl.NewManager(ctrl.GetConfigOrDie(), ctrl.Options{
		Scheme:             scheme,
		MetricsBindAddress: metricsAddr,
		Port:               9443,
	})
	if err != nil {
		setupLog.Error(err, "unable to start manager")
		os.Exit(1)
	}

• Update the manager.yaml file with:

...
    spec:
      containers:
      - command:
        - /manager
        env:
        - name: POD_NAME
          valueFrom:
            fieldRef:
              fieldPath: metadata.name
...

• Add rbac permission to the controllers

+kubebuilder:rbac:groups=core,resources=pods,verbs=get;list

• Run make manifests

Start with the linked project migration guide, especially the migrate-maingo section, and review the main.go and manager.yaml examples in the issue. Document the leader-election setup steps, link to the new documentation from the migration guide, and run make manifests to verify the documented workflow.