doc(undergrad): Link the affine group (#10797)
This is the statement `group (P₁ ≃ᵃ[k] P₁)`. Per the "Undergrad TODO trivial targets" wiki page, this should [match wikipedia](https://en.wikipedia.org/wiki/Affine_group), which says:
> In mathematics, the affine group or general affine group of any affine space over a field K is the group of all invertible affine transformations from the space into itself.
I guess you could also ask for the statement that `(P₁ ≃ᵃ[k] P₁) ≃ units (P₁ →ᵃ[k] P₁)`, but I'll PR that separately.
Co-authored-by: Patrick Massot <patrickmassot@free.fr>
