Documentation for FasterAI updates

[lorenzoh]

With the new FasterAI API, some changes should be made to the documentation.

ToDos

• the simple examples for training a model should be replaced by their short, 5-line versions.
  • [x] README.md
  • [x] Colab
  • [x] quickstart.md should use just the short versions
  • [x] serialization.ipynb
  • [x] introduction.md should pull apart the 5-liner and explain the slightly longer versions
• the task-specific tutorials should be moved into the "intermediate" category:
  • [x] image segmentation
  • [x] image keypoint regression
• [x] how-tos for training should be condensed into a single notebook
• some documentation sections need to be added:
  • [x] a tutorial on using the high-level query functions
  • [x] a tutorial on working with BlockMethods
• The following need editing:
  • [x] datacontainers.md: add high-level examples as motivation, maybe implement a recipe
  • [x] apicomparison.md