Release 4.0.0 alpha

• Big improvement of the documentation:
  • Re-structure the entire doc, remove deprecated stuff, etc.
  • Add a nice mkdocs template
  • Docs are now hosted at otbtf.readthedocs.io
  • Add a new section on the python API (end-to-end tutorial)
  • A lot of new sections: distributed training, etc...
• Refactoring all the python classes to enforce pep8
• Ubuntu version: 22.04
• Cuda version: 12.1.0
• Tensorflow version: 2.12.0
• Fixed Tensorflow error "Cannot register 2 metrics with the same name" + new test
• Faster CI build thanks to bazel remote cache
• /home/otbuser/.local/bin added to user path