Do not mention "Manually creation" of the 404 HTML

Author: humitos

docs/guides/custom-404-page.rst

@@ -10,32 +10,17 @@ As the ``404.html`` will be returned for all the URLs where the real page was no
 all its resources URLs and links must be absolute (start with a `/`),
 otherwise they will not work when a user clicks on them.
 
-In case you want to follow the same style for the 404 page than your theme, you can either:
-
-1. manually copy the source of any already rendered pages or,
-1. use the `sphinx-notfound-page`_ extension
-
-
-Manually creation
------------------
-
-Once your docs are built, you can open any of the page already built and copy its source code,
-make all the links absolute and replace the content of the body with the one you like.
-
-.. warning::
-
-   This method requires knowledge of HTML and some knowledge of how to build the URLs properly to work under Read the Docs,
-   considering that the docs are usually served under ``/{language}/{version}/``.
-
-After that, you have to define `html_extra_path`_ setting in your Sphinx's ``conf.py`` to include the ``404.html`` file created in the output.
+In case you want to follow the same style for the 404 page than your theme, you can use the `sphinx-notfound-page`_ extension.
 
 
 Using ``sphinx-notfound-page`` extension
 ----------------------------------------
 
 The ``sphinx-notfound-page`` extension helps you to create and automatically arrange all the URLs and file location without worry about them.
 You can define ``notfound_body`` setting in your Sphinx's ``conf.py`` with the content of the page.
+See it's documentation_ for more customization.
 
 
 .. _sphinx-notfound-page: https://github.com/humitos/sphinx-notfound-page
+.. _documentation: https://github.com/humitos/sphinx-notfound-page
 .. _html_extra_path: http://www.sphinx-doc.org/en/stable/usage/configuration.html#confval-html_extra_path