Fix mobile nav obstructing content

[philg314]

Instead of adding the height of .mobile-nav at the top of .document it is now added at the top of body.

Closes #95

[hugovk]

(Rebased on main to retrigger the CI and create an RTD build preview: https://python-docs-theme-previews--96.org.readthedocs.build/en/96/)

[hugovk]

Testing locally with the banner enabled on the 3.8 branch:

diff --git a/Doc/tools/templates/layout.html b/Doc/tools/templates/layout.html
index 98ccf42248..0151a00593 100644
--- a/Doc/tools/templates/layout.html
+++ b/Doc/tools/templates/layout.html
@@ -1,13 +1,11 @@
 {% extends "!layout.html" %}
 
 {% block header %}
-{%- if outdated %}
 <div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;">
     {% trans %}This document is for an old version of Python that is no longer supported.
     You should upgrade, and read the {% endtrans %}
     <a href="/3/{{ pagename }}{{ file_suffix }}">{% trans %} Python documentation for the current stable release{% endtrans %}</a>.
 </div>
-{%- endif %}
 {% endblock %}
 
 {% block rootrellink %}

Before

After

[pradyunsg]

Thanks for the screenshots on this @hugovk! ^.^