Clarify configuring airgapped custom PolicyServers vs PolicyServer default

[viccuad]

When installing via airgap, one needs to configure the PolicyServer default. This PolicyServer CR is owned by the Helm chart kubewarden-defaults, which makes the docs muddy.

When Configuring PolicyServers CA or imagePullSecrets:

• For custom self-deployed PolicyServers the topic is in https://docs.kubewarden.io/howtos/policy-servers/custom-cas#custom-certificate-authorities
• For the default PolicyServer managed by the helm chart, the topic is in https://docs.kubewarden.io/howtos/policy-servers/private-registry#consuming-the-secret-in-helm-charts
• The airgap docs doesn't mention the configuration of the PolicyServer default https://docs.kubewarden.io/howtos/airgap/install

Acceptance criteria

• The how-to airgap install doesn't have repeated code between "normal" install and "hauler" install. Each should document untl the airgap is crossed, then have a new additional airgap page documenting the needed way to install/configure the helm charts or custom PolicyServers.
• The howto/configuring-policy-servers should be clearer when dealing with either custom or the default PolicyServer. Maybe explain each of the topics as they are in separate pages, but ensure that each page has a final section, detailing the .Values needed for doing the same for the PolicyServer default.