Document recent platform features

[mintlify]

Updated documentation to reflect recent features added to the Pawtograder platform, including discussion board enhancements, assignment management dashboards, and grading improvements. This update covers 10+ new features from recent staging commits.

Files changed:

• students/discussion-board.mdx - Added leaderboard, karma system, and error pins
• staff/discussion.mdx - Added karma/leaderboard controls and error pins configuration
• staff/assignments/intro.mdx - Added assignment dashboards, score capping, and date picker for extensions
• staff/assignments/grading-assignments.mdx - Added lab leaders assignment mode
• staff/assignments/autograding.mdx - Added enhanced autograder rerun feature
• staff/assignments/handgrading.mdx - Added grading annotations improvements
• staff/gradebook.mdx - Added export grades loading state

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