move overrides.nix to overlays

also update overrides docs
it is a pure devos overlay now, it makes sense to go in overlays

doc/concepts/overrides.md

@@ -2,30 +2,27 @@
 Each NixOS host follows one channel. But many times it is useful to get packages
 or modules from different channels.
 
-This is what the overrides are for. You can make use of the `overrides.nix` to
-override specific packages to be pulled from other channels. Any overlay may get
-`channels` as their first argument.
+## Packages
+You can make use of `overlays/overrides.nix` to override specific packages in the
+default channel to be pulled from other channels. That file is simply an example
+of how any overlay can get `channels` as their first argument.
 
-## Example
+You can add overlays to any channel to override packages from other channels.
 
-### Packages
-The override packages are defined as a regular overlay with an extra arguement
-`channels`. This refers to all channels defined in `flake.nix`.
-
-Pulling the manix package from the latest flake:
+Pulling the manix package from the `latest` channel:
 ```nix
 channels: final: prev: {
   inherit (pkgs.latest) manix;
 }
 ```
 
-### Modules
+## Modules
 
 You can also pull modules from other channels. All modules have access to the 
 `modulesPath` for each channel as `<channelName>ModulesPath`. And you can use
 `disabledModules` to remove modules from the current channel.
 
-Pulling the zsh module from the latest flake:
+Pulling the zsh module from the `latest` channel:
 ```nix
 { latestModulesPath }: {
   modules = [ "${latestModulesPath}/programs/zsh/zsh.nix" ];

flake.nix

@@ -45,7 +45,6 @@
             (devos.lib.pathsIn ./overlays)
             ./pkgs/default.nix
             pkgs.overlay # for `srcs`
-            ./overrides.nix # from "latest" channel
             nur.overlay
           ];
         };