Update documentation

Summary

With the project progress, documentation has to be x-checked and updated.

What is the expected correct behavior?

• Installation instructions must be updated
  • Developers installation must be done with pip locally with mamba handling env dependencies
  • User installation must be done with pip from PyPI with mamba handling env dependencies (if not installed already)
• Use of external data sources for atmospheric calibrations should be documented (general description, requirements to have an account, etc.)
• All code docstrings must adhere to numpydoc standard (see #40 (closed))
• User manual must be added
  • UC diagrams must be included in the doc
  • Examples must be added and properly documented
  • Support channels must be added

Relevant logs and/or screenshots