Fix deployment: Extract tar before upload

[Kaes3kuch3n]

The download artifact action uncompresses the artifact, but it doesn't extract the contained tar archive automatically, so we have to do this manually before we can upload the extracted files.

See also output of test run at https://github.com/Coderdojo-Schoeneweide/website/actions/runs/24466742834/job/71496019357

[majdkt]

Copilot recommends this to avoid manual untaring:

Use the intended deploy action (recommended): actions/deploy-pages in the deploy job. It understands the Pages artifact format and doesn’t require you to manually untar.

[Kaes3kuch3n]

Copilot recommends this to avoid manual untaring:

Use the intended deploy action (recommended): actions/deploy-pages in the deploy job. It understands the Pages artifact format and doesn’t require you to manually untar.

The issue with that is that we want to deploy to a separate repository, so we cannot use the default actions/deploy-pages action. Unfortunately the one we use instead doesn't seem to support automatic untaring :(