Guide about custom 404 pages

docs/guides/custom-404-page.rst

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+Use custom ``404 Not Found`` page on my project
+===============================================
+
+If you want your project to use a custom page for not found pages instead of the "Maze Found" default one,
+you can put a ``404.html`` at the top level of your project's HTML output.
+
+When a 404 is returned, Read the Docs checks if there is a ``404.html`` in the root of your project's output and uses it if it exists.
+
+As the ``404.html`` will be returned for all the URLs where the real page was not found,
+all its resources URLs and links must be absolute (starting with a ``/``),
+otherwise they will not work when a user clicks on them.
+
+In case you don't want to deal with these links manually,
+or you want to use the same style as your theme,
+you can use the `sphinx-notfound-page`_ extension.
+
+
+Using ``sphinx-notfound-page`` extension
+----------------------------------------
+
+The ``sphinx-notfound-page`` extension automatically creates the proper URLs for your 404 page.
+Once the extension is installed, it will generate the default 404 page for your project.
+See its documentation_ for how to install and custom it.
+
+
+.. _sphinx-notfound-page: https://pypi.org/project/sphinx-notfound-page
+.. _documentation: https://sphinx-notfound-page.readthedocs.io/