nixos/documentation: deprecate docbook option docs

following the plan in https://github.com/NixOS/nixpkgs/pull/189318#discussion_r961764451

also adds an activation script to print the warning during activation
instead of during build, otherwise folks using the new CLI that hides
build logs by default might never see the warning.

nixos/modules/misc/documentation.nix

@@ -357,6 +357,14 @@ in
     (mkIf cfg.nixos.enable {
       system.build.manual = manual;
 
+      system.activationScripts.check-manual-docbook = ''
+        if [[ $(cat ${manual.optionsUsedDocbook}) = 1 ]]; then
+          echo -e "\e[31;1mwarning\e[0m: This configuration contains option documentation in docbook." \
+                  "Support for docbook is deprecated and will be removed after NixOS 23.05." \
+                  "See nix-store --read-log ${builtins.unsafeDiscardStringContext manual.optionsJSON.drvPath}"
+        fi
+      '';
+
       environment.systemPackages = []
         ++ optional cfg.man.enable manual.manpages
         ++ optionals cfg.doc.enable [ manual.manualHTML nixos-help ];