doc: _templates: show correct version in sidebar in case of releases

Ensure that version indicator in sidebar is correct in case of releases.

Fixes zephyrproject-rtos/zephyr#91799.

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

doc/_templates/zversions.html

@@ -2,7 +2,7 @@
   <div class="rst-versions" data-toggle="rst-versions" role="note" aria-label="versions">
     <span class="rst-current-version" data-toggle="rst-current-version">
       <span class="fa fa-book"> Zephyr Project</span>
-      v: latest
+      v: {{ current_version if is_release else "latest" }}
       <span class="fa fa-caret-down"></span>
     </span>
     <div class="rst-other-versions">