Write model.md

[jessegreenberg]

We can actually probably give this a first pass now, the model seems stable enough IMO.

TODO model description

[jessegreenberg]

I wrote an initial draft for this. Assisted by Claude, but I took turns to significantly reduce verbocity, remove tables, remove general knowledge/physics descriptions, and write it in the style of https://github.com/phetsims/totality/blob/main/quadrilateral/doc/model.md.

@brettfiedler @marlitas can you please review for correctnes?

If you would like to edit this file or re-write in another style, that is also fine and please feel free to do so!

[marlitas]

I restructured some things and changed wording so that the documentation focused more on the learning goals, limitations, and simplifications of the sim rather than explaining the photoelectric effect concept. I'm sure there's still a lot we can keep doing here, and more to fill in (I'm sure code review will have thoughts). Would be great to get @brettfiedler's eye on it next.