document/export chebvandermonde

This function might be useful, e.g. for fitting in different norms.

e.g. [see this discourse discussion](https://discourse.julialang.org/t/multivariate-polynomial-regression-of-discrete-data-in-l-infinity-norm/125369/7?u=stevengj)

Start by locating the chebvandermonde function and the package's documentation or export declarations. Determine how it should be documented and made publicly available, then verify that the generated documentation exposes the function and describes its use for fitting in different norms.

Scritto dal modello di indicizzazione a partire dal testo della issue.