Add Fast DDL principle and best practices (WIP)

[Benjamin2037]

补充 Fast DDL 部分新的原理，及最佳实践部分。

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[benmaoer]

LGTM

[ti-chi-bot]

@Benjamin2037: 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	c592d4c	link	true	/test pull-verify

Full PR test history. Your PR dashboard.

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