MedMNIST tutorial

[itrushkin]

Introduces the real tutorial with an opportunity of editing the code and interacting with the notebook. Tasks:

• [ ] Prepare the working sample
  • [x] Test locally
  • [ ] Test in Google Colab
• [ ] Describe the process in README
• [ ] Show OpenFL features
  • [ ] Show fine-tuning
  • [ ] Show model loading and validation
  • [ ] Show images and predictions for validation

[skip ci]