Re-check execution, length, and relevance of function examples from pkgdown website

[Rekyt]

Some functions have very long examples that we could probably shorten like: https://frbcesab.github.io/funbiogeo/reference/fb_format_species_traits.html

while others have non-explicit examples (almost all coverage functions): https://frbcesab.github.io/funbiogeo/reference/fb_count_sites_by_species.html

And even some non-running examples: https://frbcesab.github.io/funbiogeo/reference/fb_plot_species_traits_completeness.html

[ahasverus]

For the long format dataset, maybe we can select a few species(five) and a few traits (three) in the raw dataset just to speed up the examples. This idea is just to illustrate the functions fb_format_*() not to reproduce the internal datasets.