IONOS(theming): add nc-theming as submodule

[printminion-co]

Summary

• activate theme with ./ooc app:enable nc_theming
• extended .github/workflows/easycloud-build.yml to build on changes of 'apps-custom/**'
  • test locally with ./<nc-docs-and-tools>/container/check_release.sh pr-38

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[github-actions]

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