docs(drafts): add GKE MIG usage section with single strategy example

Add explanation of GKE's default single strategy approach where MIG
partition size is specified via nodeSelector while requesting
nvidia.com/gpu: 1, contrasting with mixed strategy's nvidia.com/mig-*
resource format.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: ppippi-dev