Update time_window maximum and add FAQ for scheduled scan execution policies

What does this MR do and why?

This MR updates the time_window configuration for scheduled scan execution policies and adds a comprehensive FAQ section to help users understand how scheduled scans work.

Changes

1. Increase time_window maximum: Updates the JSON schema to allow time_window values up to 2629746 seconds (approximately 30 days), increased from the previous 86400 seconds (24 hours) limit.

2. Add FAQ section: Adds a new "Frequently asked questions about scheduled scans" section that covers:

   • Maximum scheduling timespan and how to configure monthly scans with 30-day distribution
   • Behavior during instance downtimes and how scans are rescheduled
   • What happens when projects are deleted
   • How to cancel running scheduled scans
   • Best practices and recommendations for large-scale deployments

Related issues

Addresses customer questions about scheduled scan execution policies at scale.

MR acceptance checklist

• Documentation updated
• Schema validation updated