[onert/python] Add documentation for minimal.py sample and relocate script

runtime/onert/sample/minimal-python/minimal.md

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+# minimal.py — Basic Inference Example
+
+Demonstrates the simplest Python-API workflow to load an NNFW model, allocate inputs, run inference, and report success.
+
+## Purpose
+
+- Load an `.nnpackage` model
+- Automatically query input tensor shapes
+- Allocate zero-filled NumPy arrays for each input
+- Perform a single inference call via `session.infer(...)`
+- Print a confirmation message
+
+## Usage
+
+```bash
+python minimal.py /path/to/your_model.nnpackage [backends]
+```
+- `/path/to/your_model.nnpackage` – path to your NNFW package or model file
+- `backends` (optional) – backend string (e.g. "cpu", "gpu"); defaults to "cpu"