Guide about custom 404 pages

Small guide mentioning how Read the Docs find the 404.html file and
suggesting sphinx-notfound-page to generate this HTML easily.

Author: humitos

docs/guides/custom-404-page.rst

@@ -0,0 +1,27 @@
+Use custom 404 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`` on the root of your project's 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 for the 404 page than your theme in an easy way,
+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.
+Once the extension is installed into Sphinx, it will generate the default 404 page for your.
+See its documentation_ for better customization.
+
+
+.. _sphinx-notfound-page: https://pypi.org/project/sphinx-notfound-page
+.. _documentation: https://sphinx-notfound-page.readthedocs.io/