Linux: add apt-based distribution notice to DE

[naltatis]

          I feel like this would be a good warning to also have in the DE version, seeing as this page specifically leans on APT for installation.

Unless we rephrase this page entirely for all Linux installations?

Originally posted by @duckfullstop in #421 (comment)

[duckfullstop]

Unless we rephrase this page entirely for all Linux installations?

To be honest, the entire setup guide for Linux and Docker probably needs rewriting - considering how heavily the Linux guide leans on apt being available, and how confusing the Docker guide is to follow along with.

Windows should probably be split out into its own page, so that we have nice, clear, simple tabs to click depending on your OS of choice.

(but I digress, this is outside of the scope of this issue)