Update documentation

[matbun]

• [x] #159
• [ ] Create a README under docs/ explaining how to manage documentation
• [x] organize RST files in subfolders reflecting the order of the navigation
• [x] Add python script for tutorial on trainer class: https://github.com/interTwin-eu/itwinai/tree/main/tutorials/distributed-ml/torch-tutorial-2-trainer-class
• [ ] Explain difference between DDP, Horovod, and DeepSpeed
• [x] Embed video tutorials

Branch: https://github.com/interTwin-eu/itwinai/tree/docs_changes

[matbun]

@VerderK could you update the docs README please?