[skip ci]/adding architecture document for velero

Merge Request

Changes

Added Architecture.md file for Velero

Preview doc: https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/velero_arch_docs/charter/packages/velero/Architecture.md

Package MR

No package modifications

For Issue

Closes https://repo1.dso.mil/platform-one/big-bang/apps/cluster-utilities/velero/-/issues/27