Document window function limits on dimensions and metrics

[mintlify]

Adds explicit callouts in the Dimensions and Metrics references explaining that:

• Dimensions cannot use window functions in their sql (dimension SQL runs in the same SELECT as the main GROUP BY, where window functions are not allowed).
• Aggregate metrics cannot use window functions in their sql (their SQL is wrapped in an aggregate function during the main GROUP BY, and SQL disallows window functions inside aggregate calls).
• Non-aggregate metrics cannot use window functions in their sql (they sit alongside aggregate metrics in the same SELECT, which runs at the GROUP BY step).
• Post calculation metrics can use window functions, because Lightdash wraps the aggregated results in an outer SELECT and evaluates them there — where OVER (...) clauses are valid.

Points users to post calculation metrics (for metric-side window logic) and table calculations (for row-level window logic on query results).

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Requested by: jess@lightdash.com via Slack
Mintlify session: slack_1786611799.895439_C0AM8D5UUR5

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