[onert-micro] Release Note for onert-micro 2.0.0 (#14118)

- Release Note for onert-micro 2.0.0

ONE-DCO-1.0-Signed-off-by: Chunseok Lee <[email protected]>

docs/release/onert-micro/2.0/release-note-2.0.0.md

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+## Release Notes for onert-micro 2.0.0
+
+### Operations updated
+
+- New ops supported : Cast, Ceil, Elu, Fill
+- New CMSIS NN accelrated ops : SVDF, Relu, Relu6
+
+### New features for on-device training
+
+- New Trainable Operation : GRU, StridedSlice
+  - limitation : You can train GRU's weights. Since input gradient is not supported now, GRU layer should be the last layer for training.
+- New Loss Function : Sparse Categorical Cross Entropy(experimental)