Add environment-level troubleshooting guide for API authentication

[goodnessnwajichukwu]

This guide helps developers troubleshoot API authentication issues that are not caused by Pi-hole itself but by environment-level factors. It covers common causes such as:

• Port conflicts
• Reverse proxy interference
• Local DNS overrides
• Container networking issues
• Firewall blocking

The goal is to reduce unnecessary debugging, improve developer experience, and prevent support tickets caused by setup issues rather than the Pi-hole API.

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[PromoFaux]

The linter is complaining about how the table was formatted.

The | and the other characters should be separated by a space

- |------|-------------|----------------|
+ | ------ | ------------- | ---------------- | 

[PromoFaux]

Thanks, useful info but it is not actually linked anywhere... it should probably go under the Pi-hole API section of the menu (see mkdocs.yml file).

docs/mkdocs.yml

Lines 132 to 135 in 99f0b2e

	- 'Pi-hole API':
	- 'Pi-hole API': api/index.md
	- 'Authentication': api/auth.md
	- 'TLS/SSL': api/tls.md