Change about page to be mounted in the web UI (#19345)

app/controllers/api/v1/instances/extended_descriptions_controller.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class Api::V1::Instances::ExtendedDescriptionsController < Api::BaseController
+  skip_before_action :require_authenticated_user!, unless: :whitelist_mode?
+
+  before_action :set_extended_description
+
+  def show
+    expires_in 3.minutes, public: true
+    render json: @extended_description, serializer: REST::ExtendedDescriptionSerializer
+  end
+
+  private
+
+  def set_extended_description
+    @extended_description = ExtendedDescription.current
+  end
+end