fix: add Docker networking instructions for Kokoro-FastAPI connection issues

[Ansh1372]

Fixes #785

Expanded the "Connection Issues from Open WebUI" section
with 3 clear options for Docker networking:

• Option 1: Docker Desktop (host.docker.internal)
• Option 2: Docker Compose (service name)
• Option 3: Docker Network (recommended for Linux)

[pr-validator-bot]

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