Add Kaigi on Rails 2025 (ruby-conferences#929)

Author: okuramasafumi

_data/conferences.yml

@@ -3009,6 +3009,15 @@
   twitter: euruko
   announced_on: 2024-11-29
 
+- name: Kaigi on Rails 2025
+  location: Tokyo, Japan
+  start_date: 2025-09-26
+  end_date: 2025-09-27
+  url: https://kaigionrails.org/2025/
+  twitter: kaigionrails
+  mastodon: https://ruby.social/@kaigionrails
+  announced_on: 2024-10-26
+
 - name: Rocky Mountain Ruby 2025
   location: Boulder, CO
   # TODO: update when dates are confirmed