feat: add nix flake

docs/flake.nix

@@ -0,0 +1,38 @@
+{
+  perSystem =
+    {
+      pkgs,
+      lib,
+      src,
+      version,
+      ...
+    }:
+    {
+      packages.docs = pkgs.stdenv.mkDerivation {
+        inherit src version;
+
+        pname = "tree-sitter-docs";
+
+        nativeBuildInputs = [
+          pkgs.mdbook
+          pkgs.mdbook-admonish
+        ];
+
+        buildPhase = ''
+          cd docs
+          mdbook build
+        '';
+
+        installPhase = ''
+          mkdir -p $out/share/doc
+          cp -r book $out/share/doc/tree-sitter
+        '';
+
+        meta = {
+          description = "Tree-sitter documentation";
+          homepage = "https://tree-sitter.github.io/tree-sitter";
+          license = lib.licenses.mit;
+        };
+      };
+    };
+}