Update docs for staging branch features

[mintlify]

Updated documentation to reflect recent features pushed to the staging branch of pawtograder/platform. This includes new office hours capabilities, grading improvements, submission file viewer enhancements, and AI assistance tools.

Files changed:

• students/office-hours.mdx - Added notifications and help request resolution sections
• staff/office-hours/intro.mdx - Added student notifications and AI assistance sections
• staff/assignments/handgrading.mdx - Added tip about editing scores after review completion
• staff/assignments/grading-assignments.mdx - Added grading status dashboard section
• students/assignments/create-submission.mdx - Added file viewer capabilities (binary files, markdown preview)
• students/discussion-board.mdx - Added drag-and-drop image upload tip
• staff/assignments/autograding.mdx - Added test insights, regrade workflow, and empty submission detection sections
• developers/mcp-server.mdx - New page documenting MCP server for AI assistance
• docs.json - Added mcp-server to developer navigation