Promote a Docker Hub README task (publish-docker-readme-task.yml) into the template

[ptr727]

Problem

The template publishes Docker images but has no task for pushing the Docker Hub repository overview (the README shown on the Docker Hub page). Docker downstreams each add their own:

• ptr727/NxWitness ships publish-docker-readme-task.yml (matrix over its images, static Docker/README.md, peter-evans/dockerhub-description, DOCKER_HUB_ACCESS_TOKEN), called from publish-release.yml.
• ptr727/ESPHome-NonRoot now mirrors it (single repo, static Docker/README.md), wired into publish-release.yml like the date-badge job. This replaced an older bespoke m4 + toolversions README generator that also relied on a separate, undocumented DOCKER_HUB_PASSWORD secret.

Suggestion

Ship a reusable publish-docker-readme-task.yml skeleton in the template (single-repo by default, documented matrix variant for multi-image repos) and call it from publish-release.yml for Docker-shipping repos. Standardizing on DOCKER_HUB_ACCESS_TOKEN (not a separate password secret) would also remove the secret-naming drift each Docker downstream currently has to rediscover.

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Surfaced while adopting the template in ptr727/ESPHome-NonRoot (see ptr727/ESPHome-NonRoot#61).

[ptr727]

Shipped publish-docker-readme-task.yml + a static Docker/README.md, wired into publish-release.yml (gated to main), standardized on DOCKER_HUB_ACCESS_TOKEN.

Addressed in #167 (merged to develop; not yet promoted to main, pending any further feedback as these changes are applied downstream).