nginx: add missing nginx.8 manpage

Without the change "man nginx" does not render any synopsis.

Closes: https://github.com/NixOS/nixpkgs/issues/258658

pkgs/servers/http/nginx/generic.nix

@@ -2,7 +2,7 @@ outer@{ lib, stdenv, fetchurl, fetchpatch, openssl, zlib, pcre, libxml2, libxslt
 , nginx-doc
 
 , nixosTests
-, substituteAll, removeReferencesTo, gd, geoip, perl
+, installShellFiles, substituteAll, removeReferencesTo, gd, geoip, perl
 , withDebug ? false
 , withKTLS ? false
 , withStream ? true
@@ -51,15 +51,17 @@ assert lib.assertMsg (lib.unique moduleNames == moduleNames)
 stdenv.mkDerivation {
   inherit pname version nginxVersion;
 
-  outputs = ["out" "doc"];
+  outputs = [ "out" "doc" ];
 
   src = if src != null then src else fetchurl {
     url = "https://nginx.org/download/nginx-${version}.tar.gz";
     inherit hash;
   };
 
-  nativeBuildInputs = [ removeReferencesTo ]
-    ++ nativeBuildInputs;
+  nativeBuildInputs = [
+    installShellFiles
+    removeReferencesTo
+  ] ++ nativeBuildInputs;
 
   buildInputs = [ openssl zlib pcre libxml2 libxslt gd geoip perl ]
     ++ buildInputs
@@ -166,6 +168,12 @@ stdenv.mkDerivation {
   preInstall = ''
     mkdir -p $doc
     cp -r ${nginx-doc}/* $doc
+
+    # TODO: make it unconditional when `openresty` and `nginx` are not
+    # sharing this code.
+    if [[ -e man/nginx.8 ]]; then
+      installManPage man/nginx.8
+    fi
   '';
 
   disallowedReferences = map (m: m.src) modules;