docs: add backups to startup folders list (immich-app#13967)

Add the check of backups that is done on immich microservice of backups folder presence

docs/docs/administration/system-integrity.md

@@ -3,7 +3,7 @@
 ## Folder checks
 
 :::info
-The folders considered for these checks include: `upload/`, `library/`, `thumbs/`, `encoded-video/`, `profile/`
+The folders considered for these checks include: `upload/`, `library/`, `thumbs/`, `encoded-video/`, `profile/`, `backups/`
 :::
 
 When Immich starts, it performs a series of checks in order to validate that it can read and write files to the volume mounts used by the storage system. If it cannot perform all the required operations, it will fail to start. The checks include: