Add OSCAL doc for Gatekeeper

Package Merge Request

Package Changes

https://repo1.dso.mil/platform-one/big-bang/apps/core/policy/-/blob/3.8.0-bb.1/CHANGELOG.md

Package MR

https://repo1.dso.mil/platform-one/big-bang/apps/core/policy/-/merge_requests/139

For Issue

Closes https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/997