*: 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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[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	7d9f34b	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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[qiancai]

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