Add documentation for inactive project deletion feature

What does this MR do and why?

Add documentation for inactive project deletion feature. The feature is implemented in MR !85689 (merged). Unless the feature MR is merged we cannot merge the documentation changes.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Visit the following pages locally.

• http://gdk.test:3005/ee/user/admin_area/settings/#repository
• http://gdk.test:3005/ee/administration/inactive_project_deletion.html
• http://gdk.test:3005/ee/user/gitlab_com/#inactive-project-deletion
• http://gdk.test:3005/ee/administration/audit_events.html#project-events

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/357376