Debian installation step-by-step

Hi @eikek , hi everyone

I have almost completed writing down a step-by-step howto for users who want to install without docker.

Would be great if you [have a look.](https://github.com/andreklug/docspell-debian) It's the first time I publish something on github so honestly I have no idea how to make it available here.

Let me know what you think and how to proceed :) I will finish it within the next few days.

cheers
André