Adds more basic info to the default 404 page (readthedocs#9656)

* Adds more basic info to the default 404 page

* tweak text: "404 error page"

* Use request.get_host to contextualize the 404 message slightly

* Update readthedocs/templates/404.html

Co-authored-by: Eric Holscher <25510+ericholscher@users.noreply.github.com>

Co-authored-by: Eric Holscher <25510+ericholscher@users.noreply.github.com>

readthedocs/templates/404.html

@@ -16,6 +16,10 @@
 {% block language-select-form %}{% endblock %}
 
 {% block content %}
+<p>{% blocktrans context "404 page" %}This is a 404 error page. The link you have followed or the URL that you entered does not exist. It might be that the documentation has been updated, or this page has been removed.{% endblocktrans %}</p>
+
+<a href="/">{% blocktrans with site_name=request.get_host|default:_("this site") context "404 page" %}Back to the front page of {{ site_name }}{% endblocktrans %}</a>
+
 <pre style="line-height: 1.25; white-space: pre;">
 
         \          SORRY            /