add hardware device descriptions to doc

most of the text is recycled and follows no particular format

doc/hardware.nix

@@ -0,0 +1,18 @@
+with import <nixpkgs> {} ;
+
+let
+  devices =
+    builtins.readDir ../devices;
+  texts = lib.mapAttrsToList (n: t:
+    let d = import  ../devices/${n}/default.nix;
+        d' = { description = "no description for ${n}"; } // d;
+    in d'.description )
+    devices;
+in
+writeText "hwdoc" ''
+  Supported hardware
+  ##################
+
+  ${lib.concatStringsSep "\n\n" texts}
+
+''