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.
  • README.md
  • Colab
  • quickstart.md should use just the short versions
  • serialization.ipynb
  • 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:
  • image segmentation
  • image keypoint regression
• how-tos for training should be condensed into a single notebook
• some documentation sections need to be added:
  • a tutorial on using the high-level query functions
  • a tutorial on working with BlockMethods
• The following need editing:
  • datacontainers.md: add high-level examples as motivation, maybe implement a recipe
  • apicomparison.md