Add Model Viewer documentation to ReadtheDocs

[blnicho]

Summary

Document the model viewer functionality in pyomo.contrib.viewer on Readthedocs.

[mrmundt]

[BWOM] This touches some of the big documentation efforts being pushed by IDAES/PrOMMiS. The gap in this knowledge is high on the list of priorities for this. This also came up recently at IDAES Tech Team Meeting (2025). @blnicho will put it on the list of priorities.