docs: define binary-only VM provisioning contract #53

Summary #

Define the supported binary-only workflow for preparing and operating macOS virtual-machine workers while keeping spa as the public command boundary.

Changes #

• Document host, restore-image, base lifecycle, compatibility, and rollback requirements.
• Add the VM provisioning guide to the DocC and site navigation.
• Record the experimental release constraints and operator responsibilities.

Testing #

• DEVELOPER_DIR=/Applications/Xcode-26.6.0.app/Contents/Developer ./scripts/generate-documentation.sh --check

Checklist #

• Generated files are up to date.
• No credentials or unintended files are included.

Change-Id: Ia8bd0477c856f010e6bcd0978140810867f9d4fa