feat: PolicyServer PodDisruptionBudget documentation.

Add documentation to describe the `minAvailable` and `maxUnavailable`
fields in the policy server spec.

Signed-off-by: José Guilherme Vanz <[email protected]>

docs/howtos/policy-servers/03-pod-disruption-budget.md

@@ -0,0 +1,72 @@
+---
+sidebar_label: Defining PodDisruptionBudget
+title: Configuring PodDisruptionBudget for PolicyServers
+description: Configuring PodDisruptionBudget for Kubewarden PolicyServers.j
+keywords: [kubewarden, kubernetes, policyservers, poddisruptionbudget]
+doc-persona: [kubewarden-operator, kubewarden-integrator]
+doc-type: [howto]
+doc-topic: [operator-manual, policy-servers, poddisruptionbudget]
+---
+
+To enhance the resilience of Kubewarden policy server deployments, two fields
+can be used: `minAvailable` and `maxUnavailable`. These fields are used by the
+Kubewarden controller to create a
+[PodDisruptionBudget](https://kubernetes.io/docs/tasks/run-application/configure-pdb/)
+(PDB) for the policy server pods, thus ensuring high availability and
+controlled eviction in case of node maintenance or scaling operations.
+
+## Understanding minAvailable and maxUnavailable 
+
+The `minAvailable` field specifies the minimum number of policy server pods
+that must be available at all times. This is crucial for maintaining the
+operational integrity of the Kubewarden policy server, ensuring that policies
+are continuously enforced without interruption. It can be defined as an integer or a
+percentage.
+
+When set, the Kubewarden controller creates a `PodDisruptionBudget` object that prevents
+voluntary disruptions from causing the number of available replicas to fall
+below this threshold. This is particularly important during operations such as
+cluster upgrades or maintenance.
+
+The `maxUnavailable` field dictates the maximum number of policy server pods
+that can be unavailable at any given time. This setting allows for a controlled
+degree of unavailability, which can be useful for performing rolling updates or
+partial maintenance without fully halting the policy enforcement mechanism. It 
+can also be defined as integer or percentage.
+
+When configured, it informs the creation of a `PodDisruptionBudget` object that limits
+the number of pods that can be voluntarily disrupted. This ensures that even
+during disruptions, a certain level of service is maintained.
+
+## Configuring minAvailable and maxUnavailable
+
+When deploying or updating the Kubewarden policy server, you can specify these
+fields in your configuration to ensure the desired level of availability. It's
+important to note that you can specify only one of `maxUnavailable` and
+`minAvailable`.
+
+``` yaml
+apiVersion: policies.kubewarden.io/v1
+kind: PolicyServer
+metadata:
+  name: your-policy-server
+spec:
+  # Other configuration fields
+  minAvailable: 2
+```
+
+This configuration ensures that either at least two policy server pods are
+available at all times.
+
+In the same way, you can specify the `maxUnavailable` field to ensure that no
+more than 30% of the policy server pods are unavailable at any given time.
+
+``` yaml
+apiVersion: policies.kubewarden.io/v1
+kind: PolicyServer
+metadata:
+  name: your-policy-server
+spec:
+  # Other configuration fields
+  maxUnavailable: "30%"
+```