Use code blocks in Sphinx docs when referring to unit model import paths

[lbianchi-lbl]

@zacharybinger I know your initial PR #1574 was merged, but there maybe potential issues down the line if we move the locations of models (as we have done on a few occasions in some cases). In your implementation, you used code blocks, but I wonder if we should've used test_code blocks so that those import lines would be tested. Then, if someone were to put up a PR and move models, the corresponding doc would fail because the import line would no longer work.

Originally posted by @adam-a-a in #1497