nixos/make-options-doc: link manpages

Add links to manpages without a link using the mapping defined in
`doc/manpage-urls.json`, as is already done for the nixpkgs and NixOS
manuals.

nixos/lib/make-options-doc/default.nix

···
         if baseOptionsJSON == null
         then builtins.toFile "base.json" "{}"
         else baseOptionsJSON;
+
+      MANPAGE_URLS = pkgs.path + "/doc/manpage-urls.json";
     }
     ''
       # Export list of options in different format.

nixos/lib/make-options-doc/mergeJSON.py

···
 import collections
 import json
+import os
 import sys
 from typing import Any, Dict, List
 
···
             )
         result[opt.name] = opt.value
     return result
+
+manpage_urls = json.load(open(os.getenv('MANPAGE_URLS')))
 
 admonitions = {
     '.warning': 'warning',
···
     def env(self, text):
         return f"<envar>{escape(text)}</envar>"
     def manpage(self, page, section):
+        man = f"{page}({section})"
         title = f"<refentrytitle>{escape(page)}</refentrytitle>"
         vol = f"<manvolnum>{escape(section)}</manvolnum>"
-        return f"<citerefentry>{title}{vol}</citerefentry>"
+        ref = f"<citerefentry>{title}{vol}</citerefentry>"
+        if man in manpage_urls:
+            return self.link(manpage_urls[man], text=ref)
+        else:
+            return ref
 
     def finalize(self, data):
         return "".join(data)