Developer manual: mention the org's appstore-build-publish workflow #15687

developer_manual/app_publishing_maintenance/release_automation.rst recommends a third-party action, while the org has its own workflow template for this.

• Mention https://github.com/nextcloud/.github/blob/master/workflow-templates/appstore-build-publish.yml as the default option, with how to add it to a repo.
• Keep the third-party action as an alternative, or drop it if the template covers everything.