v1.31.1: Updated provisioning guide for A3 VM family

[tpdownes]

What's Changed

The A3 provisioning guide was updated by @tpdownes to support 2 use cases:

• user-created reservations without compact placement policies that are automatically consumed by matching VMs
• Google Cloud-created reservations that must be specifically identified by Slurm cluster for consumption

See #2420 and reservation consumption documentation for details.

Full Changelog: v1.31.0...v1.31.1

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This discussion was created from the release v1.31.1: Updated provisioning guide for A3 VM family.