util/nixshell: Add a Nix shell for building documentation

Add a Nix shell config allowing to build the coreboot documentation.

Change-Id: I1c9715c677342241b78fbdef0afeb4536f48d50f
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/62203
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

util/nixshell/documentation.nix

@@ -0,0 +1,13 @@
+with import <nixpkgs> {};
+
+stdenvNoCC.mkDerivation {
+	name = "coreboot-documentation";
+
+	buildInputs = [
+		git
+		gnumake
+		python3Packages.recommonmark
+		python3Packages.sphinx_rtd_theme
+		sphinx
+	];
+}