Merge pull request NillionNetwork#146 from NillionNetwork/docs/add-retention-policy-to-index

docs: add storage retention to the admin-guide contents

Author: psofiterol

docs/admin-guide.md

@@ -12,6 +12,7 @@ This section provides task-oriented instructions for node administrators.
 - [Upgrades](#upgrades)
 - [Database migrations](#database-migrations)
 - [The admin api](#the-admin-api)
+- [Storage Retention](#storage-retention)
 
 ## Configuration