Periodic checking of the up to date status of key articles

[likidu]

Is your feature request related to a problem? Please describe.
We found some key articles such as the benchmark result of TPC-H: https://docs.pingcap.com/tidb/stable/v4.0-performance-benchmarking-with-tpch were very outdated. There might be other similar docs that need to be monitored so that we can update them more regularly.

Describe the solution you'd like
We need to:

1. Have a list of those articles we want to monitor.
2. Use script to automatically report those stalled articles.

[lilin90]

Previously, for documents that we didn't want to put it in TOC but still wanted to let it accessible to those who kept those links, we just kept them in the repo without adding them to TOC. As time goes by, some documents are no longer needed or outdated, and we do need to remove them.

Progress:

• Done: Added a script that identifies the .md files that are not listed in the TOC files of TiDB and TiDB Cloud documentation @qiancai : https://github.com/pingcap/docs/blob/master/scripts/report_md_files_not_in_toc.py We can do this check quarterly.
• Next step: review the identified files to decide whether to remove them. For example, soon we'll remove all 28 benchmark files in https://github.com/pingcap/docs/tree/master/benchmark and only retain three files listed in TOC.md.