Fix duplicate notebook numbers in user guide (two at 48, four at 50)

[brendancol]

Several user guide notebooks share the same prefix number:

• 48_Sieve_Filter.ipynb and 48_Visibility_Analysis.ipynb are both numbered 48
• Four notebooks are all numbered 50: 50_COG_Overview_Generation.ipynb, 50_Polygonize_Simplification.ipynb, 50_Resample.ipynb, 50_Polygon_Clipping.ipynb

Recent PRs each picked the next available number without checking what had already merged.

Alphabetical file listings show these in the wrong order or grouped together when they shouldn't be.

Fix: renumber from 48 onward by commit date:

Current	New	Notebook
48	48	Sieve_Filter (unchanged)
48	49	Visibility_Analysis
49	50	KDE
50	51	Resample
50	52	COG_Overview_Generation
50	53	Polygonize_Simplification
50	54	Polygon_Clipping