add unstable iso image download

[flyfloh]

This MR adds the download links to the unstable image again.

I chose to add an unstable ISO link to the NixOS sidebar on the download page (see screenshot below).

If this is confusing, we could also add a separate section for the unstable downloads. I am not 100% sure what the best way to approach this is.

Fixes #635.

[thilobillerbeck]

Looks alright, though I'd opt we should put this in the ISO image tab and not in a separate one, since this kind of breaks the semantics of the download page now. Since the tabs list download types and "unstable" is not a type of NixOS image and rather a channel, so I do not really see a reason to separate into two things that are in the sense of the tab group the same.

[thilobillerbeck]

Notes from the meeting

• split the ISO image sections into Graphical and Minimal
• having a section for either stable and unstable downloads

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2025-03-03-marketing-team-minutes/61110/1