[FEATURE] Improve installation documentation

[kuelteabmas]

🚩 Problem Description

Is your feature request related to a problem?

After a successful installation, iGotify-Notification-Assistent receives messages from my gotify server (verified it in the logs) but fails to send them via APN even with the correct SECNTFY_TOKEN from my iGotify iOS app.
The issue was related to my router running PFSense not having ports 5223 and 443 opened in order to allow APN notification to be received.

Language: English or German

Example:
"I'm often frustrated when …"

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💡 Proposed Solution

Add a note in the wiki to set a firewall rule to allow traffic through ports 5223 and 443. 5223 could be sufficient but add 443 just in case.

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🔄 Alternatives Considered

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📸 Mockups / Screenshots (optional)

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🎯 Use Case

Describe how this feature would be used and who would benefit from it.

During installation

✔️ Checklist

Please confirm the following before submitting:

• I searched existing issues and feature requests
• This feature request is not a duplicate
• This request describes a single, clear feature

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ℹ️ Additional Context

Add any other context or information about the feature request here.

[kuelteabmas]

@androidseb25, If I can wiki editing permissions, I will gladly add it the instructions guide.