docs: add the word 'official' (GH-26849)

(cherry picked from commit af4a2dc)

Co-authored-by: Ned Batchelder <[email protected]>

Author: miss-islington

Doc/tools/templates/indexcontent.html

@@ -5,7 +5,7 @@
 {% block body %}
   <h1>{{ docstitle|e }}</h1>
   <p>
-  {% trans %}Welcome! This is the documentation for Python {{ release }}.{% endtrans %}
+  {% trans %}Welcome! This is the official documentation for Python {{ release }}.{% endtrans %}
   </p>
   <p><strong>{% trans %}Parts of the documentation:{% endtrans %}</strong></p>
   <table class="contentstable" align="center"><tr>