Add additional documentation for implementation of strata vs geostrata

[brandynlucca]

Add additional documentation for implementation of strata vs geostrata including how they are defined, and how and when they are used in the workflow.

[brandynlucca]

This should include specific contexts for when each are used.

[brandynlucca]

Looking over this now, I think this was already done when I had did the initial overhaul of the docs post-xarray-migration. The downstream documentation also specifically highlights how the strata and geostrata are linked for things like apportioning the kriged population estimate (via the mesh_biodata_link argument that is used throughout). So I think this can be closed?

[leewujung]

I created #454 to address this more explicitly so that people can easily search for "strata" and "geostrata" and see them in the doc page.

[leewujung]

Closing this via #434.