KAFKA-20049: Publish Hugo base image under Apache namespace

[Owen-CH-Leung]

This PR removes the short-term dependency on the personal hvishwanath/hugo image namespace and moves the repo toward an Apache-owned GHCR image.

Changes:

• update the Hugo base image reference in Makefile to ghcr.io/apache/kafka-site/hugo:v0.123.7-ext-multiplatform
• add a new GitHub Actions workflow to build and publish that Hugo base image from Dockerfile.multiplatform
• update the Docker image workflow to use the Apache-owned image, with a local build fallback if the image is not published yet
• update the README to reflect the new build flow

This is intended as the short-term fix only: preserve the current custom Hugo image behavior, but stop relying on a personal image.

Longer term, we should evaluate replacing this custom Kafka-owned Hugo image with the official Hugo image from the Hugo project, moving any repo-specific bootstrap steps into CI as needed.