Document ALTER COLUMN TYPE behaviour on parquet partitions

[javier]

Summary

Adds a "Parquet partitions" section to the ALTER TABLE COLUMN TYPE reference documenting the behaviour introduced in Enterprise 3.3.4 (OSS engine, questdb/questdb#7024):

• Type conversions now work on tables with parquet partitions, matching native-partition results.
• Lazy by default: partitions are not rewritten; decode moves to the query path, and an O3 merge materialises the new type.
• Eager rewrite for conversions involving SYMBOL and for chained conversions where the on-disk parquet type no longer matches metadata.

[github-actions]

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