Adding reference documentation

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Summary

This MR brings over some of the documentation from MFIX-Exa.

Additional information (optional)

Checklist

The proposed changes:

• fix a bug or incorrect behavior
• add new capabilities or features
• changes answers in the test suite to more than roundoff level
• are likely to affect the results of downstream users
• include documentation in the code

Release acknowledgement

In order for a merge request to be accepted, each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense. (Supporting evidence required.) ~ ~ ~ ~ ~