Critical documentation is missing

[Sbozzolo]

Foundamental concepts in ClimaCore have no documentation, especially user documentation.

As per our documentation policies, we should have documentation so that "a technically versed programmer who is not an expert in the subject matter of the code to understand the design and purpose of a code and to use it."

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1. AxisTensors have no documentation #1994
   bug documentation +2
2. The MatrixFields module has no user documentation #1985
   bug documentation +2