[Docs] Add UCM all configurable parameters reference

[ljccsu]

What does this PR do?

Add comprehensive documentation of all UCM configurable parameters.

Contents

• Top-level YAML parameters (17 params: ucm_connectors, enable_event_sync, use_layerwise, etc.)
• ucm_connector_config sub-dict (25 params: store_pipeline, storage_backends, io_direct, etc.)
• store_health health breaker config (5 params)
• Store C++ Config structures (CacheStore, PosixStore, MooncakeStore, Ds3fsStore, Compressor, FakeStore, PcStore)
• Registered pipelines (10 combos)
• Sparse attention configs (GSAOnDevice, ESA, KVStarMultiStep, Blend, ReRope)
• Metrics configuration (7 top-level keys + 9 MetricDefinition fields)
• Environment variables (UCM log, vLLM switches, MindIE, build, system-level)
• MindIE ucm_config.json (12 keys)

Checklist

• Added new documentation file
• No code changes