docs(pickers): add example to set default layout for pickers

[pkos98]

Description

Hello there, thanks for snacks 😋.

Even though the usage can generally be inferred by type annotations, many people still struggle to set the default layout for a specific picker, as illustrated by a quick search from the search engine of your choice:

https://www.reddit.com/r/neovim/comments/1ie8yv1/how_to_set_correctly_default_parameters_for/ https://www.reddit.com/r/neovim/comments/1ig6nf2/change_snacksnvim_picker_layout/ https://www.reddit.com/r/neovim/comments/1ihkkp8/anyone_know_how_to_make_snackspickerexplorer_pane/

So I think it's good to show a basic example.