[DOC-10610]: GCP backup instructions improvements.

[RayOffiah]

This change covers additional instructions for setting up a cloud backup on GCP.

You can view a preview at:

https://preview.docs-test.couchbase.com/doc-10610/server/current/manage/manage-backup-and-restore/manage-backup-and-restore.html

(If you need a user and password to access the preview, you will find them here: https://couchbasecloud.atlassian.net/wiki/spaces/DOCS/pages/1971224949/Docs+Team+demo+site+passwords)

There's still a bit of work to do, but I want to see if I'm on the right track.