Have a worked through example with individual level dataset

[Rekyt]

I'm getting a lot of question of confused users regarding the use of fundiversity with datasets at the individual level because we keep referring to "site-species" matrix and "species-trait" matrix.
Even though we've written it in the paper and in the introduction vignette, it seems that we need to specify it elsewhere with a well chosen example (maybe in another dedicated vignette?).