Improve readthedocs documentation

[JakobBD]

• installation instructions:
  • how to run the examples
  • how to include local flodym source code as a package
    • using uv
    • using PYTHONPATH (check if possible)
• add more detail on the FlodymArray dimensions caveat
• explain recommended input data format
• explain what checks are performed on input data if from_df is used.
• How to add additional fields to subclass (like a config dict)
  • how to do this if created from class method