Add example for a custom material that uses Transmission / Depth textures

## What problem does this solve or what need does it fill?

It is not obvious how to use transmission and depth textures in your own transmissive material.

This is needed for some distortion effects, water, etc.

## What solution would you like?

Example for of such usage would be very helpful.

## What alternative(s) have you considered?

I was trying to check how `StandardMaterial` is doing transmission but it has way to much code and is not easy to follow.

## Additional context

None.

Start by tracing how StandardMaterial handles transmission, since the issue identifies it as the relevant entry point but notes that it is difficult to follow. Add a focused example of a custom transmissive material using transmission and depth textures, and consider distortion or water as usage contexts; done when the example clearly shows how to apply both textures.

Written by the indexing model from the issue text.