Make a derivation for the docs

Author: michaelpj

docs/_static/.gitkeep

@@ -2,14 +2,14 @@
 
 - Code evaluation codelens ([Tutorial](https://github.com/haskell/haskell-language-server/tree/master/plugins/hls-eval-plugin/README.md)):
 
-   ![Eval Demo](../plugins/hls-eval-plugin/demo.gif)
+   ![Eval Demo](https://github.com/haskell/haskell-language-server/tree/master/plugins/hls-eval-plugin/demo.gif)
 
 - Type information and documentation on hover. Note that currently, in order for docs to be displayed for dependencies, they must have been built with GHC's `-haddock` flag:
 
   - For cabal:
       - Add to your global config file (e.g. `~/.cabal/config`):
 
-        ```cabal
+        ```
         program-default-options
           ghc-options: -haddock
         ```
@@ -18,7 +18,7 @@
 
   - For stack, add to global `$STACK_ROOT\config.yaml`, or project's `stack.yaml`:
 
-    ```cabal
+    ```
     ghc-options:
       "$everything": -haddock
     ```

docs/_templates/.gitkeep

@@ -193,6 +193,15 @@
         });
         pythonWithPackages = pkgs.python3.withPackages (ps: [ps.sphinx myst-parser sphinx_rtd_theme ps.pip]);
 
+        docs = pkgs.stdenv.mkDerivation {
+          name = "hls-docs";
+          src = pkgs.lib.sourceFilesBySuffices ./docs [ ".py" ".rst" ".md" ".png" ".gif" ".svg" ];
+          buildInputs = [ pythonWithPackages ];
+          # -n gives warnings on missing link targets, -W makes warnings into errors
+          buildPhase = ''sphinx-build -n -W . $out'';
+          dontInstall = true;
+        };
+
         # Create a development shell of hls project
         # See https://github.com/NixOS/nixpkgs/blob/5d4a430472cafada97888cc80672fab255231f57/pkgs/development/haskell-modules/make-package-set.nix#L319
         mkDevShell = hpkgs:
@@ -253,6 +262,9 @@
           haskell-language-server-8104 = mkExe ghc8104;
           haskell-language-server-8105 = builtins.throw "GHC 8.10.5 is not available in nixpkgs";
           haskell-language-server-901 = mkExe ghc901;
+
+          # docs
+          docs = docs;
         };
 
         defaultPackage = packages.haskell-language-server;