Add conditional formatting to big value docs

[mintlify]

Summary

• Added a new Conditional formatting section to the big value chart docs page
• Documents how to add color rules, define conditions (numeric and string operators), use multiple rules with priority ordering, and set separate light/dark mode colors
• Based on feature introduced in lightdash/lightdash#20383

[github-actions]

🤖 Documentation Bot

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[github-actions]

🤖 Documentation Bot

✅ All checks passed! No issues found.