Add an example for ".active" attribute in tabsets recipe?

[maelle]

👋 here, I was re-reading https://bookdown.org/yihui/rmarkdown-cookbook/html-tabs.html (for supporting it in pkgdown r-lib/pkgdown#1694) and was thinking it might make sense to add an explicit example for "By default, the first tab is active (i.e., displayed). If you want a different tab to be displayed initially, you may add the attribute .active to it." just in case readers got a little bit confused?

[cderv]

Thanks for the suggestion. If you want to do a PR you're welcome ! 😉

Otherwise, I'll add an example later. thanks!