This PR adds documentation for the new AI Assistant PDF upload feature:

1. Created a new Dashboard settings page with comprehensive information on the PDF upload feature
2. Updated API reference introduction to include PDF document upload endpoints
3. Updated AI ingestion documentation with the new PDF reference documents feature
4. Updated navigation structure in docs.json to include the new Dashboard settings page

The new feature allows customers to upload PDFs via the Dashboard that will become part of the AI Assistant's knowledge base, enabling more accurate responses based on information not necessarily included in the documentation itself.