[Maffooch]

Description

Adds docs/content/asset_modelling/PRO__archive.md, the page for the Pro Archive feature:

• what archiving an Organization, Asset, Engagement, Test or Finding does, and that everything under it goes with it;
• where archived data stops appearing (lists, search, dashboards, metrics, reports, the API, deduplication, reimport, Triage Engine rules);
• the forced retention period (6 months by default, DD_ARCHIVE_RETENTION_MONTHS on self-hosted installs) and the purge that follows it, which also removes the audit history;
• archiving from the UI, restoring from the Archived page, and the /api/v2/ actions;
• Triage Engine retention rules with the Archive Findings and Delete Findings Permanently actions.

The feature ships behind the Archive feature flag. Merge together with the Pro change so the page is not live before the feature.