Document recent staging features

[mintlify]

Updated documentation to reflect recent features pushed to the staging branch of pawtograder/platform. Added comprehensive documentation for new dashboards, office hours improvements, discussion board enhancements, and assignment features.

Files Changed

• staff/assignments/handgrading.mdx - Added global error pinning feature
• students/office-hours.mdx - Added Get Help button and rich notifications
• staff/office-hours/intro.mdx - Added office hours status cards, queue schedules, and demo queues
• staff/assignments/grading-assignments.mdx - Added Grading Status Dashboard
• students/discussion-board.mdx - Added drag-and-drop image uploads and karma system
• staff/discussion.mdx - Added drag-and-drop image uploads and karma system
• staff/assignments/intro.mdx - Added extended due date picker, optional leaderboard, and score capping
• staff/assignments/autograding.mdx - Added enhanced rerun autograder feature
• staff/assignments/rubrics.mdx - Added note about grading annotations usability improvements
• staff/assignments/dashboards.mdx - NEW: Created comprehensive dashboards page covering security, test insights, and assignment overview
• docs.json - Added dashboards page to navigation

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