[TEST PR/Don't approve] doc: warn against truncating mz_now()

[sidsaw-mz]

Summary

• Adds a warning to the mz_now() idiomatic SQL docs about the anti-pattern of using date_trunc() or similar truncation on mz_now()
• Truncating mz_now() causes batch processing instead of incremental pre-computation, defeating Materialize's core value

Motivation

From Slack discussion:

You shouldn't use MZ now with a truncated datetime, this should be considered a code-smell... mz_now on a truncated date should be considered an anti-pattern, since it literally causes a batch of data... it means we're not pre-computing

Test plan

• Preview the docs site to verify the warning renders correctly

https://claude.ai/code/session_019eLFNUkcgdEP7gaEKmLUtd

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