Add outdated version warning banner for older documentation

[bupd]

When users land on older documentation versions (via search engines, old links), they should see a prominent warning banner indicating they are viewing outdated docs.

Similar to how Kubernetes does it: https://v1-29.docs.kubernetes.io/docs/home/

The banner should:

• Display at top of page for any non-latest version
• Clearly state doc version is outdated
• Provide direct link to same page in latest docs
• Use proper banner styling (framed, visible)

Previous attempt: PR #656 was closed. Maintainer feedback suggested:

• Needs proper banner styling with frame
• Look at other Hugo themes for inspiration (k8s, knative)
• Consider making it a reusable banner component for announcements

Reference: #656

/label good first issue