Docs: unify 'manual' with 'guides' folder

This is one of the final steps towards strictly following
https://diataxis.fr/

docs/site/guides/adding-machines.md

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+
+Clan has two general methods of adding machines:
+
+- **Automatic**: Detects every folder in the `machines` folder.
+- **Declarative**: Explicit declarations in Nix.
+
+## Automatic registration
+
+Every folder `machines/{machineName}` will be registered automatically as a Clan machine.
+
+!!! info "Automatically loaded files"
+
+    The following files are loaded automatically for each Clan machine:
+
+    - [x] `machines/{machineName}/configuration.nix`
+    - [x] `machines/{machineName}/hardware-configuration.nix`
+    - [x] `machines/{machineName}/facter.json` Automatically configured, for further information see [nixos-facter](https://clan.lol/blog/nixos-facter/)
+    - [x] `machines/{machineName}/disko.nix` Automatically loaded, for further information see the [disko docs](https://github.com/nix-community/disko/blob/master/docs/quickstart.md).
+
+
+
+## Manual declaration
+
+Machines can also be added manually under `buildClan`, `clan.*` in flake-parts or via [`inventory`](../guides/inventory.md).
+
+!!! Note
+    It is possible to use `inventory` and `buildClan` together at the same time.
+
+=== "**Individual Machine Configuration**"
+
+    ```{.nix}
+    buildClan {
+        machines = {
+            "jon" = {
+                # Any valid nixos config
+            };
+        };
+    }
+    ```
+
+=== "**Inventory Configuration**"
+
+    ```{.nix}
+    buildClan {
+        inventory = {
+            machines = {
+                "jon" = {
+                    # Inventory machines can set tags
+                    tags = [ "zone1" ];
+                };
+            };
+        };
+    }
+    ```