Add RubyConf 2026 (#1070)

Author: marcoroth

_data/conferences.yml

@@ -3169,3 +3169,13 @@
   twitter: helvetic_ruby
   date_precision: year
   mastodon: https://ruby.social/@helvetic_ruby
+
+- name: RubyConf 2026
+  location: United States # TODO: update once location is confirmed
+  start_date: 2026-12-31 # TODO: update once dates are confirmed
+  end_date: 2026-12-31 # TODO: update once dates are confirmed
+  announced_on: 2024-05-07
+  url: https://rubycentral.org/news/announcing-railsconf-2025-and-a-new-chapter-for-ruby-central-events
+  twitter: rubyconf
+  date_precision: year
+  mastodon: https://ruby.social/@rubyconf