Orphaned tiup documentation pages

[kolbe]

Error Report

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

1. What is the URL/path of the document related to this issue?

https://docs.pingcap.com/tidb/v4.0/tiup-command-mirror-publish

2. How would you like to improve it?

All documentation pages should have a parent in the left-hand sidebar.

We should have a process to identify any pages that are orphaned.