versioning: 优化建议

[shawn0915]

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.

   建议将这个页面 https://cn.pingcap.com/tidb-release-support-policy/ 的部分内容，合并入这个页面 https://docs.pingcap.com/zh/tidb/stable/versioning
   比如，

   PingCAP 会提供最多至 3 年时间的常规版本更新

   以及，建议在文档中也体现EOL时间线（目前只有发布时间线）

2. Describe your suggestion or addition.

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

   ref. https://asktug.com/t/topic/1021302