fix: pin Antora UI bundle to stable version — remove snapshot:true

Problem

antora-playbook.yml configures the UI bundle with snapshot: true:

ui:
  bundle:
    url: https://gitlab.com/antora/antora-ui-default/-/jobs/artifacts/HEAD/raw/build/ui-bundle.zip?job=bundle-stable
    snapshot: true

snapshot: true tells Antora to re-download the bundle on every build, always fetching the absolute latest CI artifact from the upstream Antora UI project. A breaking change in the upstream UI could silently break the documentation site without any change to this repository.

This issue is closely related to #2 (Update template to a Red Hat template). Once the Red Hat template is adopted, the bundle URL should be pinned to a specific tagged release.

Proposed Solution

1. Resolve issue #2 first (adopt Red Hat branded Antora UI template)
2. Pin the bundle URL to a specific stable/tagged release version
3. Remove snapshot: true

If staying on the default Antora UI in the interim, replace the HEAD artifact URL with a pinned release URL, e.g.:

ui:
  bundle:
    url: https://github.com/antora/antora-ui-default/releases/download/v1.0.0/ui-bundle.zip

Files to Change

• antora-playbook.yml

Start by reading issue #2 and the UI bundle configuration in antora-playbook.yml, since the proposed Red Hat template affects which release should be pinned. Update the bundle URL to a specific stable or tagged artifact and remove snapshot: true. Done means the documentation build uses a fixed upstream bundle rather than fetching HEAD on every build.