Major documentation overhaul

### Added

• append function to co.Sequential.
• Production-ready docstrings for public functions.
• reduce_max to Reduce.

### Changed

• Rename Unity to Identity to follow torch.nn.
• Major overhaul of README, improving descriptions and adding benchmark.
• Major overhaul of docs, improving descriptions and adding benchmark.
• MHA warnings to only log once.

Removed

• Unused parameters batch_first and bidirectional for RNN, GRU, and LSTM.