docs: Add metadata architecture design for Redis and CRD backends

Document the current Redis-based metadata storage and propose
Kubernetes CRD alternative:

- Current Redis keys: mx:model:{name}, mx:models, mx:nixl_ready:*
- Data schemas for ModelMetadataRecord, WorkerMetadata, TensorDescriptor
- Workflow diagrams for source/target coordination
- Proposed ModelMetadata CRD with status subresource
- TensorDescriptors ConfigMap for large data (etcd limits)
- Implementation components: CRD definition, server changes, client changes
- Comparison table: Redis vs CRD tradeoffs
- Migration path for gradual adoption

Author: KavinKrishnan