Adds custom 404 with cowsay image (#57422)

* Adds custom 404 with image, updates CI reqs, sets version latest for TOC links on 404 page

docs/docsite/rst/404.rst

@@ -0,0 +1,12 @@
+:orphan:
+
+*****
+Oops!
+*****
+
+This page does not exist in this version of the Ansible documentation.
+
+.. image:: images/cow.png
+   :alt: Cowsay 404
+
+Return to the :ref:`nearest table of contents in this version <ansible_documentation>` or use the navigation at left to explore our latest release.

docs/docsite/rst/conf.py

@@ -45,7 +45,7 @@ AUTHOR = 'Ansible, Inc'
 # They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 # TEST: 'sphinxcontrib.fulltoc'
-extensions = ['sphinx.ext.autodoc', 'sphinx.ext.intersphinx', 'pygments_lexer']
+extensions = ['sphinx.ext.autodoc', 'sphinx.ext.intersphinx', 'pygments_lexer', 'notfound.extension']
 
 # Later on, add 'sphinx.ext.viewcode' to the list if you want to have
 # colorized code generated too for references.
@@ -211,6 +211,19 @@ html_use_opensearch = 'https://docs.ansible.com/ansible/latest'
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'Poseidodoc'
 
+# Configuration for sphinx-notfound-pages
+# with no 'notfound_template' and no 'notfound_context' set,
+# the extension builds 404.rst into a location-agnostic 404 page
+#
+# default is `en` - using this for the sub-site:
+notfound_default_language = "ansible"
+# default is `latest`:
+# setting explicitly - docsite serves up /ansible/latest/404.html
+# so keep this set to `latest` even on the `devel` branch
+# then no maintenance is needed when we branch a new stable_x.x
+notfound_default_version = "latest"
+# makes default setting explicit:
+notfound_no_urls_prefix = False
 
 # Options for LaTeX output
 # ------------------------

test/runner/requirements/sanity.txt

@@ -7,6 +7,7 @@ pylint ; python_version >= '3.5' # pylint 2.0.0 and later require python 3+
 pytest
 rstcheck ; python_version >= '2.7' # rstcheck requires python 2.7+
 sphinx
+sphinx-notfound-page
 straight.plugin  # needed for hacking/build-ansible which will host changelog generation
 virtualenv
 voluptuous ; python_version >= '2.7' # voluptuous 0.11.0 and later require python 2.7+