docs(dev): mention org appstore-build-publish workflow template

[whoalin1]

Summary

release_automation.rst only documented a third-party App Store publish action, while the Nextcloud organization already ships an appstore-build-publish workflow template.

• Recommend https://github.com/nextcloud/.github/blob/master/workflow-templates/appstore-build-publish.yml as the default option.
• Document how to add it via Actions → New workflow → organization template (or by copying the YAML).
• Keep the existing R0Wi third-party example as an alternative when the org template does not fit (it is scoped to nextcloud-releases by default).

Closes #15687

Test plan

• Sphinx build of the developer manual succeeds for the release automation page
• New Organization workflow template section links the org template and lists the Actions UI steps
• Existing third-party workflow example remains under Alternative: third-party action

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AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.