Add `group_split()` on the reference page on the website

[etiennebacher]

Close #7043

[olivroy]

It seems that this is intentional #6657. Don't know how their thinking evolved over the last 1.5 year

[etiennebacher]

There's now a clearly visible "Experimental" badge next to this kind of functions on the reference page (on the dev website) so I think it's worth including it

[MikeJohnPage]

While the documentation for group_split is not available on the reference page, it is linked to in the details section of group_map.

This creates some inconsisency in the documentation (which this PR would fix).

[DavisVaughan]

It was purposeful to not include this due to tidyr::nest(.by =) being a better replacement for this particular function. We still believe that to be true

See https://github.com/tidyverse/dplyr/issues/6639