docs: update intro notebook

[officialankan]

Cleaned up the intro notebook (1_intro_gwrefpy.ipynb).

• added a proper problem statement up front — what gwrefpy actually solves (telling influence apart from natural groundwater variation) instead of jumping straight into models and wells.
• trimmed a lot of the verbose/repetitive text and fixed some typos.
• made the Model/Well terms stand out consistently, and turned the hidden "Well object" dropdown into a visible note so it doesn't get missed.
• tidied the code cells to be more pythonic and match our formatting rules (double quotes, spacing, etc.).
• fixed a data bug where all four wells were loading the same obs1 CSV — now each well loads its own file, so the summary actually shows different stats.
• corrected the delete_well description (it takes Well objects, not names).

this closes #109

[officialankan]

@eriktoller can you look at this? checkout and look at the notebook

[eriktoller]

Builds fine on my PC and looks good, ready to merge