Remove dependency on the Planetary Computer in the docs

[gadomski]

We've always seen intermittent errors when running our example notebooks against the Planetary Computer, and another just occurred: https://github.com/stac-utils/pystac-client/actions/runs/20153827573/job/57851997703?pr=858#step:6:119. I think, for the sake of long term maintainability, that it would be a good idea to re-write any docs that use the Planetary Computer to use another API source.

In this day-and-age, nothing is forever, but government STAC endpoints (e.g. USGS's Landsat STAC server) could be considered more "neutral" and stable than commercially-supported endpoints.

[krishnaglodha]

I can help in this, should I work on it ?

[gadomski]

Help is always welcome, thanks!