High-level explanation of how stackstac aligns input data

[gjoseph92]

Users sometimes ask, "how do the geotiffs get aligned into one array"? If you read the stackstac.stack docstring many times, you could probably get a sense of this, but there should be a high-level explanation of the process on the docs.

Specifically, it's good to clarify that stackstac isn't magically aligning/coregistering the GeoTIFFs to each other in some way. It's just picking a reasonable common grid for the items (resolution, CRS, bounds), and then warping each input item to match that grid.

xref #108 (reply in thread)

[gjoseph92]

Should probably include/mention #152 in this as well.