Clarification of geometry and material documents

[peterfpeterson]

This split out the material information into a new concept page, added additional links between the various docs, and added a big note at the top of SetSample that the dict definitions are in cm.

To test:

Build the docs and decide that they are better than they were.

Fixes #21467

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Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

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[peterfpeterson]

I added the (Li7) example to the docs. I didn't know that was an issue, but it definitely benefits from the clarification.

[martyngigg]

I'm happy with the new structure - it gives more clarity for the concepts and algorithms that should be used.