Flesh out Blueprint protocol documentation for mixed-vars / material-dependent fields

[BradWhitlock]

The documentation for making mixed-variables and material-dependent fields is very sparse. It would be helpful to have examples of these things, similar to what was done for the various matset flavors.

• The docs indicate that vertex-associated material fields are possible - what would that look like??
• Can we store vectors in mixed representations? What does it look like?

fields:
  unibuffer:
    topology: main
    matset: mat
    values:
      x: []
      y: []
    matset_values:
      x:[]
      y:[]

fields:
  multibuffer:
    topology: main
    matset: mat
    matset_values:
      A:
        x:[]
        y:[]
      B:
        x: []
        y: []