Add Ruby Conf Africa 2026 (ruby-conferences#1227)

* Add Ruby Conf Africa 2026

Details added;
- name
- location
- start_date
- end_date
- url
- twitter
- announced_on
- cfp_open_date
- cfp_close_date
- cfp_link

* Ruby Conf Africa 2026 should be after Baltic Ruby 2026

Author: banta

_data/conferences.yml

@@ -3328,6 +3328,17 @@
   cfp_close_date: 2025-12-31
   cfp_link: "https://www.papercall.io/balticruby2026"
 
+- name: Ruby Conf Africa 2026
+  location: Nairobi, Kenya
+  start_date: 2026-08-21
+  end_date: 2026-08-22
+  url: https://rubyconf.africa/
+  twitter: african_ruby
+  announced_on: 2025-11-06
+  cfp_open_date: 2025-11-20
+  cfp_close_date: 2026-03-11
+  cfp_link: https://www.papercall.io/ruby-conf-africa-2026
+
 - name: Rails World 2026
   location: Austin, TX
   start_date: 2026-09-22