theme: reword version warning

Clearer phrase, now that we're paving the road to have "/stable/" and
"/latest/" URLs, where stable is the latest stable version that has been
released, and latest is the last development commit (typicaly from the
"master" or "main" branch)

Inspired by the banners that appear in the Jinja documentation:

latest: https://jinja.palletsprojects.com/en/latest/
Warning: This is the development version. The latest stable version is
Version 3.0.x.

old: https://jinja.palletsprojects.com/en/2.11.x/
Warning: This is an old version. The latest stable version is Version
3.0.x.

docs/common/version-warning.html

@@ -50,10 +50,17 @@
     }
 </style>
 
+<!--
+TODO: Improvement: when we change "latest" to point to the development version,
+the message could be changed to this:
+    Warning: You are reading docs for a development version.
+    GO TO LATEST STABLE VERSION
+-->
+
 <div id="version-warning" class="alert" style="display: none">
-    <span class="warning-text">Documentation of an old version</span>
+    <span class="warning-text"><strong>Warning:</strong> You are reading docs for an old version.</span>
     <div class="div-btns">
-        <span><a id="link-to-stable" class="alert-btn">GO TO LATEST VERSION</a></span>
+        <span><a id="link-to-stable" class="alert-btn">GO TO LATEST STABLE VERSION</a></span>
     </div>
 </div>