Add documentation and default FF for Scheduled PEP experiment

What does this MR do and why?

This MR adds documentation for the Scheduled Pipeline Execution Policy (PEP) experiment feature that will be introduced in GitLab 18.0. The documentation explains how this new experimental feature allows users to enforce custom CI jobs on a scheduled cadence (daily, weekly, or monthly) across projects, independent of commit activity.

In the same MR we are enabling this experimental feature by default (although user needs to enable the experiment manually as stated in the documentation).

Key additions include:

• Adding a reference to Scheduled PEP in the policies index page
• Creating a new comprehensive documentation page that covers:
  • Feature overview and use cases
  • Configuration instructions with YAML examples
  • Schedule types (daily, weekly, monthly) with examples
  • Snoozing functionality
  • Requirements and limitations
  • Troubleshooting guidance

This documentation is important as it will help users understand and implement this new security policy type that enables regular security scans and compliance checks even when there are no new commits to a repository.

References

Related to [backend] Add documentation pipeline execution ... (#504142 - closed)