Remove Docker, add server migration checklist to readme

- Remove virtualisation.docker.enable (no longer used after native
  migration of Authelia, go2rtc, and nginx)
- Add migration checklist documenting which state directories and
  secrets to back up when moving to new hardware

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hosts/FredOS-Mediaserver.nix

@@ -18,9 +18,6 @@
       yt-dlp
     ];
 
-    # Enable Docker
-    virtualisation.docker.enable = true;
-
     # Basic networking
     networking.useDHCP = lib.mkDefault true;