Add bulk-processing section to downtime credits workflow

Summary

This MR adds a new "Bulk Processing for Large-Scale Downtime Events" section to the downtime credits workflow to handle situations where one or more downtime events in a month affect a significant number of customers (100+).

Changes

• Added new section between the main workflow steps and FAQ section
• Documented when to use bulk processing (100+ affected customers)
• Outlined 5-step procedure:
  1. Gather comprehensive event information
  2. Determine affected customers with Support leadership and Engineering
  3. Create Support Response issue for pre-approval
  4. Optionally create event-specific macro for streamlined responses
  5. Process multiple tickets efficiently using Zendesk bulk update functionality
• Included important considerations emphasizing individual validation is still required

Related Issues

Addresses the need for a streamlined process during large-scale GitLab.com downtime events affecting many customers.

Checklist

• Documentation follows existing workflow structure
• Links to relevant resources (Support Response template, macro creation workflow, Zendesk bulk update docs)
• Maintains consistency with existing validation requirements