build docs with newer nixos

no reason to keep carrying out custom graphviz package or build all this old
junk just for the docs

docs.nix

-{ pkgs ? import <nixpkgs> { } }:
-let
-  # NixOS 19.03 packaged graphviz has trouble rendering our architecture
-  # overview.  Latest from upstream does alright, though.  Use that.
-  make-graphviz = (import (pkgs.path + /pkgs/tools/graphics/graphviz/base.nix) {
-    rev = "b29d8e369011b832f72e0d250a05a0a15dcb5daa";
-    sha256 = "1w61filywn9cif2nryf6vd34mxxbvv25q34fd34am1rx70bk08ps";
-    version = "b29d8e369011b832f72e0d250a05a0a15dcb5daa";
-  });
-  graphviz = (pkgs.callPackage make-graphviz { }).overrideAttrs (old: {
-    patches = [];
-  });
-in
-  pkgs.callPackage ./privatestorageio.nix { inherit graphviz; }
+{ pkgs ? import ./stable2105.nix { } }:
+pkgs.callPackage ./privatestorageio.nix { }

shell.nix

 let
   nixpkgs = import (builtins.fetchTarball (builtins.fromJSON (builtins.readFile ./nixpkgs.json))) { };
-  stable2105 = import (builtins.fetchTarball (builtins.fromJSON (builtins.readFile ./nixpkgs-2105.json))) { };
+  stable2105 = import ./stable2105.nix { };
 in
 { pkgs ? nixpkgs }:
 pkgs.mkShell {

stable2105.nix

+import (builtins.fetchTarball (builtins.fromJSON (builtins.readFile ./nixpkgs-2105.json)))