move details on channels into the dedicated page

[fricklerhandwerk]

Motivation

reduce the noise, make each page more focused on its main topic

Context

• remove noise from the top of nix-channel page
  replace the long blurb with a short summary of what the command does,
  link to the details in the dedicated page
• slightly reword the descriptions of the files involved for clarity
• reorder the sections in the channels page to match the order of
  interaction: you first subscribe to channels and then store channels contents

Priorities and Process

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[roberth]

We can confidently state that this is a problem for reproducibility and make it slightly more actionable.

Suggested change

	> The state of a subscribed channel is external to the Nix expressions relying on it.
	> This may limit reproducibility.
	> The state of a subscribed channel is external to the Nix expressions relying on it.
	> This limits reproducibility, and may be a reason to use a lock file instead of channels.

I would like to link a resource that helps with deciding the kind of lock file, but I don't want to derail this review, so I'd prefer to settle for an incremental improvement here, as far as my input is concerned. I'd be happy to accept a suggestion.

[fricklerhandwerk]

We'd be opening up quite the rabbit hole here. I generally agree with the direction, but my aspiration would be to point people to The Right Way of doing things, and that to my knowledge still doesn't exist. The current version says something that is at least not wrong: use fetchers explicitly.

[roberth]

I guess a pin is not necessarily in a lock file. Fair enough.
We could confidently say that pinning and/or locking is The Right Way, but it might be a distraction.

Initially I thought nix.dev had a suitable article about this, but they seem to be about specific solutions, which is good for tutorials; was looking for a guide.

Let's not get ourselves stuck on this and fill this gap later.

[roberth]

Suggested change

	> Dependencies on other Nix expressions can be declared explicitly with:
	> - [`fetchurl`](@docroot@/language/builtins.md#builtins-fetchurl), [`fetchTarball`](@docroot@/language/builtins.md#builtins-fetchTarball), or [`fetchGit`](@docroot@/language/builtins.md#builtins-fetchGit) in Nix expressions

This is going off topic. The answer to "how do I fetch?" should probably not be on the Channels page, and mention all options, not just the eval-time fetchers.

• nix-channel
• FODs, best fed to derivations
• builtin fetchers, best fed to import and friends

[fricklerhandwerk]

There's NixOS/nix.dev#750 to address this, which needs some focus time to write down cleanly. I agree this would not be about channels but essentially a concept page.

[roberth]

Suggested change

	- `$XDG_STATE_HOME/nix/profiles/channels` for regular users
	- `$NIX_STATE_DIR/profiles/per-user/root/channels` for `root`
	- profile `$XDG_STATE_HOME/nix/profiles/channels` for regular users
	- profile `$NIX_STATE_DIR/profiles/per-user/root/channels` for `root`

[fricklerhandwerk]

I'm not sure what this is supposed to achieve 🤔