doc: fix book.toml not being used, make linkcheck optional

lib/book.toml

@@ -1,9 +1,10 @@
 [book]
-title="@projectName@"
+title = "@projectName@"
 
 [output.html.print]
 enable = false # don't encourage people to create more dead trees
 
 [output.linkcheck]
+optional = true
 follow-web-links = true
 warning-policy = "error"

lib/doc.nix

@@ -27,17 +27,14 @@
 
   mkMdBook = { projectName, moduleDoc }: with pkgs; stdenv.mkDerivation {
     name = "${projectName}-docs";
-    nativeBuildInputs = [
-      mdbook
-      # mdbook-linkcheck # obiously doesn't work in sandbox
-    ];
+    nativeBuildInputs = [ mdbook ];
     buildCommand = ''
       mkdir src
 
       cp ${pkgs.substituteAll {
         src = ./book.toml;
         inherit projectName;
-      }} src/book.toml
+      }} book.toml
       echo -e "# Summary\n\n- [Options](options.md)" > src/SUMMARY.md
       ln -s ${moduleDoc}/options.md ./src