intro Poly in Saturn

[WenrongZou]

No description provided.

[TentativeConvert]

I don't think we need an additional level here. Could you please do the following instead:

• replace MvPolynomial by Polynomial in levels 3 and 4
• move the explanations about MvPolynomisl, Fin n and multiple variables into a new hint on the first line of the proof of level 5
• in level 3, add explanations for Polynomial instead