docs: describe if exists / if not exists clauses for views

[jepett0]

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Description for reviewers

Docs for:

• Views: if exists / if not exists for DDL #10831

Issues:

• views: IF EXISTS / IF NOT EXISTS to help with scripting view dropping / creation #10818
• docs: add description for new syntax: create view if not exists and drop view if exists #15621

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🟢 2025-03-12 09:41:11 UTC The validation of the Pull Request description is successful.

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⚪ 2025-03-12 09:41:47 UTC Pre-commit check linux-x86_64-relwithdebinfo for 6f165d4 has started.
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Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
0	0	0	0	0	0

🟢 2025-03-12 09:44:23 UTC Build successful.