migration-guide (#2717)

This PR adds a migration guide for existing NixOS configurations.

Co-authored-by: Pablo Ovelleiro Corral <mail@pablo.tools>
Reviewed-on: https://git.clan.lol/clan/clan-core/pulls/2717
Co-authored-by: pinpox <clan@pablo.tools>
Co-committed-by: pinpox <clan@pablo.tools>

docs/site/manual/migration-guide.md

@@ -0,0 +1,175 @@
+# Migrate existing NixOS configurations
+
+This guide will help you migrate your existing Nix configurations into Clan. Keep
+in mind, though, this approach can be trickier and might lead to bugs or
+unexpected issues. We recommend following the Getting Started guide first. Once
+you have a working setup, you can easily transfer your Nix configurations over.
+
+!!! Danger
+    **Back up your existing configuration!**
+    Before you start, it is strongly recommended to back up your existing
+    configuration in any form you see fit. If you use version control to manage
+    your configuration changes, it is also a good idea to follow the migration
+    guide in a separte branch until everything works as expected.
+
+
+## Starting Point
+
+We assume you are already using NixOS flakes to manage your configuration. If
+not, migrate to a flake-based setup following the official [NixOS
+documentation](https://nix.dev/manual/nix/2.25/command-ref/new-cli/nix3-flake.html)
+The snippet below shows a common Nix flake. For this example we will assume you
+have have two hosts: **berlin** and **cologne**.
+
+```nix
+{
+  description = "My NixOS systems";
+
+  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+  outputs = { self, nixpkgs, ... }: {
+
+    nixosConfigurations = {
+
+      berlin = nixpkgs.lib.nixosSystem {
+        system = "x86_64-linux"; # TODO should we use nixpkgs.hostPlatform here too?
+        modules = [./machines/berlin/configuration.nix];
+      };
+
+      cologne = nixpkgs.lib.nixosSystem {
+        system = "x86_64-linux";
+        modules = [./machines/cologne/configuration.nix];
+      };
+    };
+  };
+}
+```
+
+## Add clan-core Input
+
+Add `clan-core` to your flake as input. It will provide everything we need to
+manage your configurations with clan.
+
+```nix
+inputs.clan-core = {
+  url = "git+https://git.clan.lol/clan/clan-core";
+  # Don't do this if your machines are on nixpkgs stable.
+  inputs.nixpkgs.follows = "nixpkgs";
+};
+```
+
+## Update Outputs
+
+To be able to access our newly added dependency, it has to be added to the
+output parameters.
+
+```diff
+-  outputs = { self, nixpkgs, ... }:
++  outputs = { self, nixpkgs, clan-core }:
+```
+
+The existing `nixosConfigurations` output of your flake will be created by
+clan. In addition, a new `clanInternals` output will be added. Since both of
+these are provided by the output of `lib.buildClan`, a common syntax is to use a
+`let...in` statement to create your clan and access it's parameters in the flake
+outputs.
+
+For the provide flake example, your flake should now look like this:
+
+```nix
+{
+  description = "My NixOS systems";
+
+  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+  outputs = { self, nixpkgs, ... }:
+  let
+    clan = clan-core.lib.buildClan {
+        directory = self; # this needs to point at the repository root
+        specialArgs = {};
+        inventory.meta.name = "NEEDS_TO_BE_UNIQUE"; # TODO: Changeme
+
+        machines = {
+            berlin = {
+                nixpkgs.hostPlatform = "x86_64-linux";
+                imports = [ ./machines/berlin/configuration.nix ];
+            };
+            cologne = {
+                nixpkgs.hostPlatform = "x86_64-linux";
+                imports = [ ./machines/cologne/configuration.nix ];
+            };
+        };
+    };
+  in
+  {
+      nixosConfigurations = clan.nixosConfigurations;
+      inherit (clan) clanInternals;
+  };
+}
+```
+
+Et voilà! Your existing hosts are now part of a clan. Existing Nix tooling
+should still work as normal. To check that you didn't make any errors, run `nix
+flake show` and verify both hosts are still recognized as if nothing had
+changed. You should also see the new `clanInternals` output.
+
+```
+❯ nix flake show
+git+file:///my-nixos-config
+├───clanInternals: unknown
+└───nixosConfigurations
+    ├───berlin: NixOS configuration
+    └───cologne: NixOS configuration
+```
+
+Of course you can also rebuild your configuration using `nixos-rebuild` and
+veryify everything still works.
+
+## Add Clan CLI devShell
+
+At this point Clan is set up, but you can't use the CLI yet. To do so, it is
+recommended to expose it via a `devShell` in your flake. It is also possible to
+install it any other way you would install a package in Nix, but using a
+developtment shell ensures the CLI's version will always be in sync with your
+configuration.
+
+A minimal example is provided below, add it to your flake outputs.
+
+```nix
+devShells."x86_64-linux".default = nixpkgs.mkShell {
+  packages = [ clan-core.packages."x86_64-linux".clan-cli ];
+};
+```
+
+To use the CLI, execute `nix develop` in the directory of your flake. The
+resulting shell, provides you with the `clan` CLI tool. Since you will be using
+it every time you interact with Clan, it is recommended to set up
+[direnv](https://direnv.net/).
+
+Verify everything works as expected by running `clan machines list`.
+
+```
+❯ nix develop
+[user@host:~/my-nixos-config]$ clan machines list
+berlin
+cologne
+```
+
+## Specify Targets
+
+Clan needs to know where it can reach your hosts. For each of your hosts, set
+`clan.core.networking.targetHost` to its adress or hostname.
+
+```nix
+# machines/berlin/configuration.nix
+{
+  clan.core.networking.targetHost = "123.4.56.78";
+}
+```
+
+## Next Steps
+
+You are now fully set up. Use the CLI to manage your hosts or proceed to
+configure further services. At this point you should be able to run commands
+like `clan machines update berlin` to deploy a host.
+