Add websocket paths to the selfhosted guide

[lixmal]

[Copilot]

Pull Request Overview

This PR enhances the selfhosted guide by adding WebSocket endpoint documentation and clarifying relay service network requirements. The changes provide clearer guidance for users setting up reverse proxies with NetBird.

• Adds WebSocket proxy paths for signal and management services
• Documents relay service QUIC/UDP requirements and proxy limitations
• Provides technical note about L4 proxy requirements for relay service

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