Add warning about empty topics in Custom Webhook filters

[oskarciupider-alchemy]

Add warning about empty topics in webhook filters

Description

Adds a block to the “How to Set Up Custom Webhooks” section to clearly explain the behavior of empty topic filters.
This is a common point of confusion for users and often results in unexpectedly high webhook volume and usage charges.
The warning links to the Event Log Filters guide for more details.

Related Issues

Changes Made

• Added a new block directly under the "How to Set Up Custom Webhooks" instructions
• Explained the behavior of empty topics (wildcard matching)
• Included documentation link for proper topic filtering
• No other content modified

Testing

• I have tested these changes locally
• I have run the validation scripts (pnpm run validate)
• I have checked that the documentation builds correctly