Improve sphinx documentation for fields.

[BradWhitlock]

The field documentation is not adequate for mixed-material fields.

I recently tried creating a mixed-material field for a dataset I was building and had some trouble. I was only able to do it successfully after calling the venn() function, saving the data to YAML, and examining the field. This took a while.

The Fields section needs to include:

• Description of material-dependent fields (what/why/how)
• Description of a mixed-material field (what/why/how)
• Provide example mixed-material field for a basic 2x2 or 3x3 zone topology + unibuffer matset (as inline YAML in the docs). Use nice graphic and not ASCII-art.
• Provide explanation of why mixed-field needs "values" and "matset_values" -- and why not just "matset_values"?
• Explain whether matset_values ever changes its composition depending on matset flavor. It looks like this happens in the venn() example. The "full" matset style for venn() makes material components under the matset_values. If this is true, explain and provide YAML examples in the docs.