Add notice to 0.4 docsite (#419)

Author: dphfox

docs/assets/overrides/main.html

@@ -31,4 +31,9 @@
 		<div class="fusiondoc-hero" style="--hero-img: url({{ page.meta.hero_img }});"></div>
 	{% endif %}
 	{{ super() }}
+{% endblock %}
+
+{% block announce %}
+The information on this page is for a future version of Fusion. Use the version
+switcher to go back to a current version of Fusion.
 {% endblock %}