docs/flake.nix: add flake outputs for docs

We have stuff that's referenced in CI but not available as flake outputs
to build. Adding default.nix outputs to flake.nix so flake users can
reference them easier.

docs/flake.lock

@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1706683685,
-        "narHash": "sha256-FtPPshEpxH/ewBOsdKBNhlsL2MLEFv1hEnQ19f/bFsQ=",
+        "lastModified": 1743938762,
+        "narHash": "sha256-UgFYn8sGv9B8PoFpUfCa43CjMZBl1x/ShQhRDHBFQdI=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "5ad9903c16126a7d949101687af0aa589b1d7d3d",
+        "rev": "74a40410369a1c35ee09b8a1abee6f4acbedc059",
         "type": "github"
       },
       "original": {

docs/flake.nix

@@ -19,7 +19,8 @@
         "x86_64-linux"
       ];
 
-      lib = nixpkgs.lib;
+      # Note, this should be "the standard library" + HM extensions.
+      lib = import ../modules/lib/stdlib-extended.nix nixpkgs.lib;
 
       forAllSystems = lib.genAttrs supportedSystems;
 
@@ -39,6 +40,8 @@
           echo "Generated ./static/style.css"
         '';
       };
+
+      releaseInfo = lib.importJSON ../release.json;
     in {
       devShells = forAllSystems (system:
         let
@@ -50,5 +53,20 @@
             packages = [ fpkgs.p-build ];
           };
         });
+
+      # Expose the docs outputs
+      packages = forAllSystems (system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+          docs = import ./default.nix {
+            inherit pkgs lib;
+            release = releaseInfo.release;
+            isReleaseBranch = releaseInfo.isReleaseBranch;
+          };
+        in {
+          inherit (docs) manPages jsonModuleMaintainers;
+          inherit (docs.manual) html htmlOpenTool;
+          inherit (docs.options) json;
+        });
     };
 }