doc: Disallow immediate deletion behind :disallow_immediate_deletion FF

What does this MR do and why?

This merge request updates GitLab's documentation to reflect changes in how project and group deletion works. The main changes include:

1. Simplified documentation structure: The separate sections for "delayed project deletion" and "delayed group deletion" have been consolidated into a single "deletion protection" section, making the documentation cleaner and easier to understand.
2. Updated default retention period: The default time before permanent deletion has changed from 7 days to 30 days, giving users more time to recover accidentally deleted items.
3. New restriction information: Added details about a new feature flag (disallow_immediate_deletion) that will prevent users from immediately deleting projects or groups once they're scheduled for deletion, removing the override option.
4. Historical updates: Updated the version history sections to accurately reflect when these features were introduced and moved between GitLab tiers.

The changes make the deletion protection feature more consistent across projects and groups while providing clearer documentation about how the system works.

Related work:

• Backend MR: backend: Disallow immediate deletion behind :di... (!201957 - merged) • Rémy Coutable • 18.4
• Frontend MR: frontend: Disallow immediate deletion behind :d... (!202040 - merged) • Rémy Coutable, Peter Hegman • 18.4

References

• Related to #561680+s

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