better advertise the `coq-mathcomp-classical`

It was observed during the MathComp-Analysis dev meeting of 2023-12-21
that the existence of the `coq-mathcomp-classical` is not well-known @Tragicus 

We should maybe improve the MathComp website
https://math-comp.github.io/installation.html
so that it is more apparent (and indeed it is not mentioned there!) @t6s 

