feat: Add support for the references in FAQs (#10699)

Currently, it’s unclear whether an FAQ item is generated from a
document, derived from a conversation, or added manually.

This PR resolves the issue by providing visibility into the source of
each FAQ. Users can now see whether an FAQ was generated or manually
added and, if applicable, by whom.

- Move the document_id to a polymorphic relation (documentable).
- Updated the APIs to accommodate the change.
- Update the service to add corresponding references. 
- Updated the specs.

<img width="1007" alt="Screenshot 2025-01-15 at 11 27 56 PM"
src="https://github.com/user-attachments/assets/7d58f798-19c0-4407-b3e2-748a919d14af"
/>

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

spec/enterprise/services/captain/llm/conversation_faq_service_spec.rb

@@ -49,10 +49,10 @@ RSpec.describe Captain::Llm::ConversationFaqService do
       it 'saves the correct FAQ content' do
         service.generate_and_deduplicate
         expect(
-          captain_assistant.responses.pluck(:question, :answer, :status)
+          captain_assistant.responses.pluck(:question, :answer, :status, :documentable_id)
         ).to contain_exactly(
-          ['What is the purpose?', 'To help users.', 'pending'],
-          ['How does it work?', 'Through AI.', 'pending']
+          ['What is the purpose?', 'To help users.', 'pending', conversation.id],
+          ['How does it work?', 'Through AI.', 'pending', conversation.id]
         )
       end
     end