Draft: Machine Learning restructuring

Machine Learning:

• Overview: software/machine_learning.md
• TensorFlow: software/tensorflow.md
• PyTorch: software/pytorch.md
• Tensorboard: software/tensorboard.md
• Distributed Training: software/distributed_training.md
• Hyperparameter Optimization (OmniOpt): software/hyperparameter_optimization.md
• PowerAI: software/power_ai.md

Closes #98 (closed), #99 (closed), #101 (closed), #103 (closed), #105 (closed), #110 (closed), #112 (closed), #118 (closed), #119 (closed), #120 (closed), #153 (closed), #154 (closed), #155 (closed), #156 (closed), #157 (closed)