Add documentation for AI Assistant PDF upload feature

[mintlify]

This PR adds documentation for the new AI Assistant feature that allows customers to include external reference information from PDFs.

Changes included:

1. Created a new Dashboard Settings page with comprehensive documentation about the PDF upload feature
2. Updated the AI Ingestion documentation to include information about how PDF references enhance AI Assistant responses
3. Updated the docs.json file to include the new Dashboard Settings page in the navigation structure

This documentation covers how customers can upload PDFs via the Dashboard that will become part of the AI Assistant's knowledge, enabling more accurate responses based on information not necessarily included in the documentation itself.