Mention edge collapse in rips documentation

[VincentRouvreau]

In Rips complex definition:

The number of simplices in the full Rips complex is exponential in the number of vertices, it is thus usually restricted, by excluding all the simplices with filtration value larger than some threshold, and keeping only the dim_max-skeleton. It may also be a good idea to start by making the point set sparser, for instance with [Gudhi::subsampling::sparsify_point_set()](https://gudhi.inria.fr/doc/3.4.1/group__subsampling.html#ga0ad8f1d6b8e3aeb1303d83ab895fdf54), since small clusters of points have a disproportionate cost without affecting the persistence diagram much.

A link to edge collapse (or an example) maybe helpfull for the end users. Same issue in the python rips documentation