docs: Updating Scoped Directory Access preview docs for DP2

Small updates to Scoped Directory Access preview docs, which include
the ability to ask for full volume access on secondary volumes,
and the "Don't ask again" checkbox in the UI for repeated access
requests.

Also, adding Scoped Directory Access to preview docs TOC.
I also updated preview/_book.yaml, even though I don't think
it is being used yet, to ensure it stays in synch with this TOC
change.

Bug: 27931961 27931982
Change-Id: Ib7d20519d580e9c7c467fdc6b2f88b699b1ff1c1