docs: vstorage reference

[dckc]

rendered:

• https://dc-vstorage-ref.documentation-7tp.pages.dev/reference/vstorage-ref.html

combined with Querying Vstorage, this

closes #758 closes #765

This is the output of an ava snapshot test, lightly edited

https://github.com/Agoric/dapp-agoric-basics/blob/dc-vstorage-ref/contract/test/test-vstorage-ref.js https://github.com/Agoric/dapp-agoric-basics/blob/fd23036/contract/test/snapshots/test-vstorage-ref.js.md

based a3p

REPOSITORY                          TAG                  IMAGE ID       CREATED       SIZE
ghcr.io/agoric/agoric-3-proposals   latest               66d5553b0fd6   8 days ago    3.3GB

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[dckc]

never mind separating this from #1016