docs: fix moduleDoc being interpreted by bash

Including the `moduleDoc` text directly in the `copy_docs` script meant
special chars were interpreted by bash.

Write the text to the nix store so we can `cat` a file instead.

docs/mdbook/default.nix

@@ -202,11 +202,12 @@ with lib; let
           + (
             if opts.index.moduleDoc == null
             then "cp ${mkMDDoc opts.index.options} ${path}"
-            else ''
+            else
+              # Including moduleDoc's text directly will result in bash interpreting special chars,
+              # write it to the nix store and `cat` the file instead.
+              ''
                 {
-              cat <<EOF
+                cat ${pkgs.writeText "module-doc" opts.index.moduleDoc}
-              ${opts.index.moduleDoc}
-              EOF
                 cat ${mkMDDoc opts.index.options}
                 } > ${path}
               ''