sphinxHook: inherit from python3Packages

doc/languages-frameworks/python.section.md

···
 ```
 
 The hook is also available to packages outside the python ecosystem by
-referencing it using `python3.pkgs.sphinxHook`.
+referencing it using `sphinxHook` from top-level.
 
 ### Develop local package {#develop-local-package}
 

pkgs/top-level/all-packages.nix

···
 
   sphinx-serve = with python3Packages; toPythonApplication sphinx-serve;
 
+  inherit (python3Packages) sphinxHook;
+
   sphinxsearch = callPackage ../servers/search/sphinxsearch { };
 
   spice = callPackage ../development/libraries/spice { };