Improve notebook self-containment and accessibility for new users

[jordanpadams]

Summary

Users have reported that the monolithic requirements.txt installs many heavyweight dependencies even when only a subset of notebooks is needed (e.g., installing spiceypy, pywwt, obspy, and geopandas just to run a simple peppi recipe). This theme covers improvements to make each notebook independently runnable with minimal setup friction.

Goals

• Reduce barrier to entry for new users running individual notebooks
• Enable notebooks to run on hosted platforms (Binder, Google Colab) without a pre-built environment
• Improve discoverability of what each notebook actually requires

Sub-Issues

Sub-issues will be linked here as they are created.