Update modeling BlackBody docs to expand use of scale keyword

### Description

The new as of v4.0 BlackBody model has the scale keyword that can be used to change the units of the returned values. This is explained in the docs for the deprecated blackbody functions. It would be good to add examples of how to get flam units (for example) instead of the default fnu units.

### Additional context

Start with the BlackBody model documentation and compare it with the documentation for the deprecated blackbody functions, where scale and flam units are already discussed. Add examples showing how to use scale to obtain flam units instead of the default fnu units, then build the documentation and check that the examples render correctly.

Written by the indexing model from the issue text.