doc: add 3.1 to /latest header note
<p class="admonition-title">Important</p>
<p>This is the latest documentation for the unstable development branch of
Project ACRN (master).<br/>Use the drop-down menu on the left to select
- documentation for a stable release such as <a href="/3.0/">v3.0</a> or
- <a href="/2.7/">v2.7</a>.</p>
+ documentation for a stable release such as <a href="/3.1/">v3.1</a> or
+ <a href="/3.0/">v3.0</a>.</p>
</div>
{% endif %}
{{ super() }}
