nixos/image: move docs into manual

nixos/doc/manual/installation/installation.md

···
 changing-config.chapter.md
 upgrading.chapter.md
 building-nixos.chapter.md
+building-images-via-systemd-repart.chapter.md
 ```

nixos/modules/image/repart.nix

···
           | tee repart-output.json
       '';
 
-    meta = {
-      maintainers = with lib.maintainers; [ nikstur ];
-      doc = ./repart.md;
-    };
+    meta.maintainers = with lib.maintainers; [ nikstur ];
 
   };
 }