nix: add user doc and pkg fixes

- Add user doc for how to use with nix/nixos

- fix potential collisions in packages if both are installed via
  `nix-env`

nix/configuration-test.nix

@@ -39,7 +39,11 @@ in
     urls = ["http://localhost:7880/api/v1/open/upload/item/blabla"];
   };
 
-  environment.systemPackages = [ pkgs.docspell.tools pkgs.jq ];
+  environment.systemPackages =
+    [ pkgs.docspell.tools
+      pkgs.docspell.server
+      pkgs.docspell.joex pkgs.jq
+    ];
 
   services.xserver = {
     enable = false;

nix/pkg.nix

@@ -17,11 +17,11 @@ in
     buildPhase = "true";
 
     installPhase = ''
-      mkdir -p $out/{bin,program}
-      cp -R * $out/program/
+      mkdir -p $out/{bin,docspell-restserver-${cfg.version}}
+      cp -R * $out/docspell-restserver-${cfg.version}/
       cat > $out/bin/docspell-restserver <<-EOF
       #!${bash}/bin/bash
-      $out/program/bin/docspell-restserver -java-home ${jre8_headless} "\$@"
+      $out/docspell-restserver-${cfg.version}/bin/docspell-restserver -java-home ${jre8_headless} "\$@"
       EOF
       chmod 755 $out/bin/docspell-restserver
     '';
@@ -39,11 +39,11 @@ in
     buildPhase = "true";
 
     installPhase = ''
-      mkdir -p $out/{bin,program}
-      cp -R * $out/program/
+      mkdir -p $out/{bin,docspell-joex-${cfg.version}}
+      cp -R * $out/docspell-joex-${cfg.version}/
       cat > $out/bin/docspell-joex <<-EOF
       #!${bash}/bin/bash
-      $out/program/bin/docspell-joex -java-home ${jre8_headless} "\$@"
+      $out/docspell-joex-${cfg.version}/bin/docspell-joex -java-home ${jre8_headless} "\$@"
       EOF
       chmod 755 $out/bin/docspell-joex
     '';