Add BitNet Embeddings 0.6B/270M I2_S conversion guide and update README

- Add docs/bitnet-embeddings-i2s-guide.md with model overview, I2_S GGUF
  conversion details, accuracy verification, inference performance benchmarks,
  and quick start examples for both 0.6B (Qwen3) and 270M (Gemma3) models
- Add embedding quantization chart (fig1_quant_per_task.png)
- Remove old docs/bitnet-embeddings-gguf-conversion.md (replaced by new guide)
- Add bitnet-embedding-0.6b and bitnet-embedding-270m to supported HF models
  in setup_env.py
- Update README.md What's New section with link to the new guide

setup_env.py

@@ -56,6 +56,12 @@ SUPPORTED_HF_MODELS = {
     "tiiuae/Falcon-E-1B-Base": {
         "model_name": "Falcon-E-1B-Base",
     },
+    "microsoft/bitnet-embedding-0.6b": {
+        "model_name": "bitnet-embedding-0.6b",
+    },
+    "microsoft/bitnet-embedding-270m": {
+        "model_name": "bitnet-embedding-270m",
+    },
 }
 
 SUPPORTED_QUANT_TYPES = {