docs(user-guide): add ai champions leaderboard documentation

[alezander86]

Summary

Adds documentation for the AI Champions Leaderboard feature in the Analytics Dashboard.

Changes

• Added ai-champions-leaderboard.md page under docs/user-guide/analytics/ with full feature description
• Added leaderboard overview screenshot (images/leaderboard-overview.png)
• Updated analytics/index.md to include the new page in the section
• Updated sidebars.ts to register the new page in navigation
• Updated glossary.md with relevant terms
• Added two FAQ articles: "What is the AI Champions Leaderboard?" and "How is my score calculated?"

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

Covers EPMCDME-11551.

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