Adding beginner friendly explanations on the Docs

[dylannelson]

Proposed new feature or change:

1) More examples/distinctions between unstructured vs structured grids

• The explanation in user-guide/terminology could be expanded to further distinguish between the two -
• Could be a separate page, in the “Why UXarray” section

2) Visual explanation and definition for cartesian vs spherical coordinates

• There's notes at user-guide/representation
• Definitions for spherical vs cartesian coordinates may benefit from a visualization or definition below it’s header. It has a Note after the table, but it could be brought up above to the body in the form of a definition

3) More distinct defintions for UxDataAray vs UxDataset

• Defined here: user-guide/data-structures
• A “UxDatAarray” is a subset of a UxDataset (ie a dataset contains a collection of data arrays)
• The current definition makes it seem as if a dataset is simply the plural of data array