Document Slack integration custom message format and template variables

[patrikbraborec]

Problem

The Slack integration docs do not document the custom message template format or the available template variables. Today the only way to figure out what variables and formatting are supported is to read the integration's source code, which is not accessible/practical for external users.

Action point

Add documentation for:

• The custom message format used by the Slack integration
• The full list of available template variables and what they resolve to
• Examples of common message templates

Reference

• https://docs.apify.com/platform/integrations/slack