[docs] setup flake and dir

docs/default.nix

@@ -0,0 +1,32 @@
+{ pkgs, lib, self, ... }:
+
+with lib;
+let
+  docbuilder = self.packages.x86_64-linux.gokill-docbuilder;
+
+  prepareMD = ''
+    # Copy inputs into the build directory
+    cp -r --no-preserve=all $inputs/* ./
+
+    ${docbuilder}/bin/docbuilder --output ./
+    substituteInPlace ./SUMMARY.md \
+    --replace "@GOKILL_OPTIONS@" "$(${docbuilder}/bin/docbuilder)"
+
+    cat ./SUMMARY.md
+  '';
+in
+pkgs.stdenv.mkDerivation {
+  name = "gokill-docs";
+  phases = [ "buildPhase" ];
+  buildInputs = [ pkgs.mdbook ];
+
+  inputs = sourceFilesBySuffices ./. [ ".md" ".toml" ];
+
+  buildPhase = ''
+    dest=$out/share/doc
+    mkdir -p $dest
+    ${prepareMD}
+    mdbook build
+    cp -r ./book/* $dest
+  '';
+}