docs: Add documentation for `LANGFUSE_WEBHOOK_WHITELISTED_*` environment variables

[csicar]

Adds documentation for these PRs

• https://github.com/langfuse/langfuse/pull/8977
• https://github.com/langfuse/langfuse/pull/9207
• https://github.com/langfuse/langfuse/pull/9205

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[!IMPORTANT] Adds documentation for LANGFUSE_WEBHOOK_WHITELISTED_* environment variables to configure webhook access and resolve security issues.

• Documentation:
  • Adds troubleshooting section in webhooks-slack-integrations.mdx for LANGFUSE_WEBHOOK_WHITELISTED_* environment variables to resolve blocked hostname/IP issues.
  • Updates configuration/index.mdx to include LANGFUSE_WEBHOOK_WHITELISTED_HOST, LANGFUSE_WEBHOOK_WHITELISTED_IPS, and LANGFUSE_WEBHOOK_WHITELISTED_IP_SEGMENTS for configuring webhook access.

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[Capricorn0010]

有配置的例子或文档吗？