*: add column-level masking policy feature document

[tiancaiamao]

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• Other reference link(s): *: add document for column-level masking policy feature docs#22613

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[qiancai]

@tiancaiamao Please involve a tech reviewer for this PR. Thanks.

[bb7133]

Please address the comments~

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[tiancaiamao]

@qiancai 这里是否特殊说明一下，CTAS 其实是没有实现的，或者叫保留给将来使用
原因不是 column masking 这边没做实现
而是说，由于我们的 DDL 那边本身是没有支持 create table as select 这样的用法
必须分成两步做，create table like + insert into select
由于没有 create table as select 的实现，column masking 也就没有实际 去实现 CTAS

[ti-chi-bot]

@tiancaiamao: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	d73e99f	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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[tiancaiamao]

address comment, PTAL @qiancai