docs: don't show doc version admonition on main

It is hard to justify keeping the version admonition/disclaimer eating
up so much real estate on all documentation pages. It was useful when
search engines tended to index old versions of the documentation and
reminding people where they "landed" was a good idea.
Now, it is just taking up space and we can safely assume that people
explicitly interested in older releases will find their way to the right
place, as our version selector is in a very similar spot to how other
projects do it.

Note that the admonition will still be included on "released" versions
of the docs, since the "there might be a more current version of this
page" admonition is actually useful to have.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/_templates/layout.html

@@ -5,12 +5,6 @@
       The <a href="/latest/{{ pagename }}.html">latest development version</a>
       of this page may be more current than this released {{ version }} version.
     </div>
-  {% else %}
-    <div class="wy-alert wy-alert-danger" data-nosnippet>
-     This is the documentation for the latest (main) development branch of
-     Zephyr. If you are looking for the documentation of previous releases, use
-     the drop-down list at the bottom of the left panel and select the desired version.
-    </div>
   {% endif %}
   {{ super() }}
 {% endblock %}