docs(freertos/smp): Add granular locks design document

The design document captures the **Dual Spinlock With Data Group Locking**
scheme used by the granular-locks implementation: data-group definitions
and hierarchy, public and port-layer API, TCB locks, deferred state
changes, ISR-only kernel critical-section helpers, event-list TOCTOU
considerations, and current limitations.

Author: Dazza0