feat(help): add comment about new template version

Added a comment at the top of rest-docs-v3.html to warn maintainers about new version of this template.

Author: nazarbodak221

cl/api/templates/rest-docs-v3.html

@@ -2,6 +2,19 @@
 {% load static %}
 {% load extras %}
 
+{% comment %}  
+╔═════════════════════════════════════════════════════════════════════════╗
+║                               ATTENTION!                                ║  
+║ This template has a new version behind the use_new_design waffle flag.  ║  
+║                                                                         ║  
+║ When modifying this template, please also update the new version at:    ║  
+║ cl/simple_pages/templates/v2_help/rest-docs-vlatest.html                ║  
+║                                                                         ║  
+║ Once the new design is fully implemented, all legacy templates          ║  
+║ (including this one) and the waffle flag will be removed.               ║  
+╚═════════════════════════════════════════════════════════════════════════╝  
+{% endcomment %}
+
 {% block title %}[Deprecated] REST API, v3.15 – CourtListener.com{% endblock %}
 {% block description %}Deprecated — REST API for federal and state case law, PACER data, the searchable RECAP Archive, oral argument recordings and more. Provided by Free Law Project, a 501(c)(3) non-profit. Please donate to support this service.{% endblock %}
 {% block og_description %}REST API for federal and state case law, PACER data, the searchable RECAP Archive, oral argument recordings and more. Provided by Free Law Project, a 501(c)(3) non-profit. Please donate to support this service.{% endblock %}