Explain MSIX staging, registration, and provisioning

[Roy (RDMacLachlan)]

Summary

• explains device-wide staging and per-user registration
• documents provisioning, package lifetime, deprovisioning, and multi-user updates
• adds current force-provisioning platform limits
• incorporates the deployment model documented by the Inside MSIX team

This PR supersedes #490 and uses the requested branch naming standard.

Reviewed for technical accuracy by Howard Bot.

Fixes AB#36938354

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