feat: add transformer/attention end-to-end example

[ulises-jeremias]

nn/layers/attention.v (6.5KB) and nn/layers/positional_encoding.v exist but are
never demonstrated in examples/.

Task

Create examples/nn_transformer_attention/ with:

1. main.v — self-attention demo with positional encoding
2. README.md — explanation of attention mechanism and how to run

Verification

• Example compiles and runs cleanly