Add the Sphinx notfound page extension (#15669)

* add sphinx notfound extension
* add notfound conf
* upgrade requirements
* use double backticks
* add urls prefix

docs/docsite/conf.py

@@ -35,9 +35,13 @@ extensions = [
     'sphinx.ext.coverage',
     'sphinx.ext.ifconfig',
     'sphinx_ansible_theme',
+    'notfound.extension',
     'swagger',
 ]
 
+notfound_urls_prefix = "/en/latest/"
+notfound_template = "404.rst"
+
 html_theme = 'sphinx_ansible_theme'
 html_theme_path = ["_static"]