nix-build: xo-arena: build docs + minor doc facepalms

pkgs/xo-arena.nix

@@ -1,12 +1,19 @@
 {
   # nixpkgs dependencies
-  stdenv, cmake, catch2,
+  lib, stdenv, cmake, catch2,
+  doxygen,
+
+  python3Packages,
+
+  sphinx, graphviz,
 
   # xo dependencies
-  xo-cmake,
   xo-randomgen,
   xo-reflectutil,
   xo-indentlog,
+  xo-cmake,
+
+  buildDocs ? false,
 } :
 
 stdenv.mkDerivation (finalattrs:
@@ -15,13 +22,29 @@ stdenv.mkDerivation (finalattrs:
 
     src = ../xo-arena;
 
-    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"
-                  "-DENABLE_TESTING=1"
-                 ];
+    cmakeFlags = ["-DCMAKE_MODULE_PATH=${xo-cmake}/share/cmake"]
+                  ++ lib.optionals buildDocs ["-DXO_ENABLE_DOCS=on"]
+                  ++ ["-DENABLE_TESTING=1"];
+
+    inherit buildDocs;
     doCheck = true;
+
+    postBuild = lib.optionalString buildDocs ''
+      cmake --build . -- docs
+    '';
+
     nativeBuildInputs = [
       cmake catch2
       xo-cmake xo-randomgen
+    ] ++ lib.optionals buildDocs [
+      doxygen
+      sphinx
+      graphviz
+      python3Packages.sphinx-rtd-theme
+      python3Packages.breathe
+      python3Packages.sphinxcontrib-ditaa
+      python3Packages.sphinxcontrib-plantuml
+      python3Packages.pillow
     ];
     propagatedBuildInputs = [
       xo-reflectutil