Document sortOptions field

Kustomize has a new field called "sortOptions" to sort resources:
kubernetes-sigs/kustomize#3913
kubernetes-sigs/kustomize#4019

Document what it does and how to use it.

Signed-off-by: Yannis Zarkadas <yanniszark@gmail.com>

site/content/en/references/kustomize/kustomization/sortoptions/_index.md

@@ -0,0 +1,105 @@
+---
+title: "sortOptions"
+linkTitle: "sortOptions"
+type: docs
+weight: 23
+description: >
+    Sort the kustomize resources.
+---
+
+The `sortOptions` field is used to sort the resources kustomize outputs.
+
+IMPORTANT:
+- Currently, this field is enforced only for the top-level kustomization.
+- This is the endorsed way to sort resources. Users should avoid using
+the CLI flag `--reorder` and instead use `sortOptions`.
+
+Currently, we support the following sort options:
+- `legacy`
+- `fifo`
+
+## Legacy Sorting
+
+In `legacy` order, kustomize sorts resources by using two priority lists:
+- An `orderFirst` list for resources which should be first in the output.
+- An `orderLast` list for resources which should be last in the output.
+- Resources not on the lists are will appear in-between, sorted by their GVK.
+
+### Example 1: Legacy Sorting with orderFirst / orderLast lists
+
+In this example, we use the `legacy` sort order to output `Namespace` objects
+first and `Deployment` objects last.
+
+```yaml
+kind: Kustomization
+sortOptions:
+  order: legacy
+  legacySortOptions:
+    orderFirst:
+    - Namespace
+    orderLast:
+    - Deployment
+```
+
+### Example 2: Default Legacy Sorting
+
+If you specify `legacy` sort order without any arguments for the lists,
+kustomize will fall back to the lists we were using before introducing this
+feature. This is useful if you want to continue using the sort order kustomize
+was using by-default before introducing this feature.
+
+These two configs are equivalent:
+
+```yaml
+kind: Kustomization
+sortOptions:
+  order: legacy
+```
+
+is equivalent to:
+
+```yaml
+kind: Kustomization
+sortOptions:
+  order: legacy
+  legacySortOptions:
+    orderFirst:
+    - Namespace
+    - ResourceQuota
+    - StorageClass
+    - CustomResourceDefinition
+    - ServiceAccount
+    - PodSecurityPolicy
+    - Role
+    - ClusterRole
+    - RoleBinding
+    - ClusterRoleBinding
+    - ConfigMap
+    - Secret
+    - Endpoints
+    - Service
+    - LimitRange
+    - PriorityClass
+    - PersistentVolume
+    - PersistentVolumeClaim
+    - Deployment
+    - StatefulSet
+    - CronJob
+    - PodDisruptionBudget
+    orderLast:
+    - MutatingWebhookConfiguration
+    - ValidatingWebhookConfiguration
+```
+
+## FIFO Sorting
+
+In `fifo` order, kustomize does not change the order of resources. They appear
+in the order they are loaded in `resources`.
+
+### Example 3: FIFO Sorting
+
+```yaml
+kind: Kustomization
+sortOptions:
+  order: fifo
+```