add saas scenario best practices

[lilinghai]

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Add TiDB best practices for SaaS scenario

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✅ Code review completed, 5 comments generated.

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Comments suppressed due to low confidence (1)

best-practices/saas-best-practices.md:23

• [nitpick] The hardware configuration recommendations for TiKV and PD appear to be duplicated (see lines 20-21 and 23-24). Consider removing the redundant entries to avoid confusion.

* TiKV：8 vCPU 32 GiB 或更高.

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@lilinghai: 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	09d136f	link	true	/test pull-verify

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