Update docs for staging branch features

[mintlify]

Updated documentation to reflect recent features and improvements from the staging branch. Added documentation for new features including AI assistance, improved office hours UX, enhanced grading workflows, and file handling improvements.

Files Changed

• staff/gradebook.mdx - Added undefined value tolerance documentation for mean calculations
• students/assignments/create-submission.mdx - Documented binary file support, markdown preview, and empty submission detection
• staff/assignments/grading-assignments.mdx - Added TA reassignment fix and grading status dashboard documentation
• staff/assignments/handgrading.mdx - Documented TA edit permissions after review completion
• students/office-hours.mdx - Added Get Help button, notifications, and help request resolution flow
• staff/office-hours/intro.mdx - Documented AI assistance MCP server and queue configuration features
• staff/assignments/autograding.mdx - Added test insights, error explorer, and regrade workflow documentation
• students/discussion-board.mdx - Enhanced drag-and-drop image upload documentation
• staff/discussion.mdx - Added drag-and-drop image upload documentation