docs(nix): import the module before enabling services.gitea-mirror (#434)

The README, NIX.md and the production snippet in the deployment guide set
services.gitea-mirror straight after adding the flake input, but the
option only exists once gitea-mirror.nixosModules.default is imported.
Only the quick start snippet showed that (#426). Every snippet that
enables the service now shows the import and where the input goes.

NIX.md

@@ -46,9 +46,13 @@ nix run --extra-experimental-features 'nix-command flakes' .#gitea-mirror
 
 ### 4. NixOS System Service
 ```nix
-# configuration.nix
+# flake.nix
+inputs.gitea-mirror.url = "github:RayLabsHQ/gitea-mirror";
+
+# configuration.nix (needs `inputs` passed through specialArgs)
+{ inputs, ... }:
 {
-  inputs.gitea-mirror.url = "github:RayLabsHQ/gitea-mirror";
+  imports = [ inputs.gitea-mirror.nixosModules.default ];
 
   services.gitea-mirror = {
     enable = true;

README.md

@@ -174,10 +174,15 @@ nix profile install --extra-experimental-features 'nix-command flakes' github:Ra
 gitea-mirror
 ```
 
-**NixOS users** - add to your configuration:
+**NixOS users** - add the flake input, import the module, then enable the service:
 ```nix
+# flake.nix
+inputs.gitea-mirror.url = "github:RayLabsHQ/gitea-mirror";
+
+# configuration.nix (needs `inputs` passed through specialArgs)
+{ inputs, ... }:
 {
-  inputs.gitea-mirror.url = "github:RayLabsHQ/gitea-mirror";
+  imports = [ inputs.gitea-mirror.nixosModules.default ];
 
   services.gitea-mirror = {
     enable = true;

docs/NIX_DEPLOYMENT.md

@@ -115,10 +115,15 @@ Access at http://localhost:4321, sign up (first user is admin), and configure ev
 
 ### Production Configuration
 
-For production with custom domain and firewall:
+For production with custom domain and firewall. Like every snippet on this
+page, it needs the module imported first; `services.gitea-mirror` does not
+exist until `gitea-mirror.nixosModules.default` is in your modules list.
 
 ```nix
+{ inputs, ... }:
 {
+  imports = [ inputs.gitea-mirror.nixosModules.default ];
+
   services.gitea-mirror = {
     enable = true;
     host = "0.0.0.0";