Add Discourse as sponsor (#2403)

Author: ericholscher

docs/_data/berlin-2025-config.yaml

@@ -208,6 +208,9 @@ sponsors:
       brand: Archbee
   publisher:
   second:
+    - name: discourse
+      link: https://discourse.org/pricing?ref=writethedocs
+      brand: Discourse
   first:
   in_kind:
     - name: readthedocs