GCP v10.0: Draft the release notes

[pose]

Have reviewed notes ready before the release is cut. Model: v9.0.0, where the release body is the migration guide.

Acceptance criteria

• Draft reviewed before the release is cut.
• Opens with a prose intro naming the version and the scope of the upgrade.
• Back-links the v9 migration guide for users coming from 8.x.
• Breaking changes are an alphabetised list keyed by resource in Pulumi casing (gcp.compute.Subnetwork), with the required user action on each.
• Any change too complex for a bullet gets its own narrative section.
• Links the v10 migration guide.
• Attached to the GitHub release page on publish.