Reorganize FAQ and troubleshoot related docs

[yikeke]

Change Request

This repository is ONLY used to solve problems related to DOCS-CN.
For other issues (TiDB, TiKV, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

https://docs.pingcap.com/zh/tidb/dev/tidb-faq
https://docs.pingcap.com/zh/tidb/dev/identify-slow-queries

FAQ 和故障诊断的内容 SW 期间加进来的时候有些乱，需要再拆分和优化

2. Describe your suggestion or addition.

3. Provide some reference materials (documents, websites, etc) if you could.

ref: Add missing tikv docs to TOC.md by Connor1996 · Pull Request #3574 · pingcap/docs-cn

[yikeke]

@kissmydb 之后会帮忙优化 FAQ 和故障诊断 的相关文档

[github-actions]

This issue is stale because it has been open for 60 days with no activity. If no comment has been made and the lifecycle/stale label is not removed, this issue will be closed in 15 days.