Improve physics documentation

That's one of the few things I do have documentation for! And yes, I think it's appropriate for a "theory manual" as long as the terminology is consistent. That said, I of course don't reference the stiffness matrix in the existing docs... https://serac.readthedocs.io/en/latest/sphinx/theory_reference/solid.html . I'll add an issue to improve this in my upcoming examples/tutorials/documentation pass.

_Originally posted by @jamiebramwell in https://github.com/LLNL/serac/pull/624#discussion_r805059366_

Start with the linked solid theory reference page and the original pull request discussion. Identify the terminology that needs consistency and determine how the stiffness matrix should be covered in the theory manual. Done means the relevant physics documentation is expanded and terminology is consistent.

Written by the indexing model from the issue text.