[Docs] Add common troubleshooting docs

### TL;DR

Add common troubleshooting docs, for example like guardian doesn't get triggered because of Guardian GitHub action is not allowed to run

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

_No response_

The issue asks for troubleshooting docs but gives no specifics. Look at existing Guardian documentation to understand its setup and common user errors. Check GitHub Actions logs or issues for recurring problems like permissions. 'Done' means a new doc section with clear steps to diagnose and fix issues like actions not running.

索引モデルが issue の本文から書いたものです。