Document how to configure the Nix channel

[Kemi-Elizabeth]

Where on docs.platform.sh should be changed?

We would like to make the configuration of the Nix channel more explicit to our users, to allow them to upgrade the channel themselves when needed.

Two things to do:

• Document which Nix channels are supported
• Document how to configure the Nix channel

All changes will be made on https://docs.platform.sh/create-apps/app-reference/composable-image.html

What exactly should be updated?

Two things to do:

• Document which Nix channels are supported
• Document how to configure the Nix channel

Additional context

https://linear.app/platformsh/issue/ADV-1961/composable-image-document-how-to-configure-the-nix-channel