Documentation for working with remodeler hook functions

[benjamin-heasly]

In RenderToolbox4, it's possible to make scene adjustments using "remodeler" hook functions in addition to or instead of a mappings file. The Flythrough example is the best example of this so far.

We should add more documentation for how to set up remodeler hooks, and how to use them. Some specific examples that would be good:

• How to remove an existing point light?
• How to assign spectra to surfaces?

The new documentation should explain:

• What hooks are available and how to assign functions to the hooks.
• The function signature required for hook functions.
• Description of remodelers that work on the mexximp scene struct.
• Description of remodelers that work on the mMitsuba or mPbrt native scene representation.
• Examples of useful operations, like the two above.