Add a troubleshooting section when the USB stick doesn't appear in the Mac startup disks
_Originally created by @sajolida on [#15127 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/15127)_
Feature Branch: doc/15127-mac-troubleshooting
Parent Task: tails/tails#14548
### Related issues
- **Blocks** tails/tails#16711
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